Here is a Secondary 1 Science question that is much more useful than it first appears: Is melting ice a chemical change? No. The ice becomes liquid water, but the chemical substance remains H₂O. Now try a second question: Is rusting iron a chemical change? Yes. Iron reacts under suitable conditions and rust contains new chemical substances. The trick is not to memorise which objects belong on which side of a table. It is to learn what changes and what stays the same.
For parents searching for a Secondary 1 Chemistry tutor in Hougang, this distinction can be a revealing starting point. The student may recognise every example in the textbook and still fail a question involving a new everyday situation. That tells us to focus on reasoning from evidence rather than handing out a longer list to memorise. In Singapore, students ordinarily study these foundations in lower-secondary Science rather than a separate national Secondary 1 Pure Chemistry paper. Topic sequence and expected detail vary with the school’s G1, G2 or G3 course.
I would begin with one question: Has the identity of the substance changed? During a physical change, the substance’s chemical identity is generally retained, even when appearance, state, shape or arrangement changes. During a chemical change, new chemical substances are formed. This is the headline answer; the rest of our work is to use observations and models carefully enough to justify it. The examples here are conceptual and paper-based; hazardous reactions and heating belong only in suitable supervised school settings.
Why “it looks different” is not a full explanation
A solid block of ice looks completely different from the puddle it leaves behind. Both are water. A piece of wood that burns changes in appearance, but this is a different kind of process: reactions produce other substances, including gases and solid residues under the relevant conditions. Looking different does not distinguish the two processes by itself.
I ask students to separate observation from interpretation. “A colour change was observed” is an observation. “A new compound formed” is a conclusion which needs context and evidence. Some chemical changes have visible clues, but not every visible clue proves a reaction; some reactions occur without a dramatic obvious sign. Learning this nuance early prevents the child from becoming a collector of simplistic exam slogans.
The question the student must answer is not “Did something exciting happen?” It is “What evidence indicates whether the chemical identities stayed the same or changed?” That question travels with the learner through later studies of reactions, rates, bonding and practical investigation.
1. Physical change: changing form without changing identity
A physical change alters a physical characteristic or state while the chemical identity of the substance remains the same. Melting ice, freezing pure water, cutting paper and crushing a piece of chalk are familiar introductory examples. The size, shape or state may change dramatically, but the chemical composition of the particular material need not change.
For ice becoming liquid water, the water molecules remain water molecules. In a simple particle model, their arrangement and freedom of movement change as the substance changes state. We do not draw each H₂O molecule splitting into hydrogen and oxygen because that does not describe ordinary melting.
A useful student answer is concise: “Melting ice is a physical change because solid water becomes liquid water without forming a different chemical substance.” This is better than saying “because it can freeze again,” though reversibility can be a useful additional observation in some contexts.
2. Chemical change: forming new substances
A chemical change involves a change in chemical identity and the formation of one or more new substances. Examples include rusting iron, burning suitable fuels, and many reactions between chemicals. Atoms may be rearranged in chemical reactions; the substances present after the reaction differ from the starting substances.
A beginner should distinguish the process from its visible signs. For rusting, the observable reddish-brown material is evidence to examine, and Science knowledge identifies new iron-containing compounds formed through reactions involving environmental conditions. The detailed chemistry can be studied later as appropriate to the class.
I would not ask a Secondary 1 learner who has not yet studied equations to memorise a complicated reaction equation. First they should be able to say that the original metal has participated in a reaction and new substances are present. When later lessons introduce chemical formulae, that reasoning gives the symbols a meaningful home.
3. Melting is not chemical decomposition
Imagine pure ice warming to liquid water. The molecules have more freedom to move relative to one another in the liquid than in the solid structure. The chemical formula remains H₂O. No chemical decomposition into hydrogen and oxygen is implied by ordinary melting.
The most common wrong answer is “it is chemical because heat is added.” Energy transfer can accompany physical changes as well as chemical processes. Heating does not automatically mean a reaction occurred. It can change temperature, state, volume or reaction rate depending on the circumstances.
I ask students to repair the sentence: “Melting is a physical change because ______.” The blank should name the unchanged substance, not merely the possibility of reversing the process. Then change the example to freezing water and ask the same question without the original wording.
4. Boiling water is not a chemical reaction
When pure water boils under appropriate conditions, liquid water changes to water vapour. The water molecules remain H₂O. Bubbles during boiling largely contain water vapour, and the formation of those bubbles does not mean the water has become a new chemical substance.
This is a perfect way to challenge the rule “bubbles always mean chemical change.” They do not. Bubbles can occur when a liquid boils or when dissolved gases leave solution as conditions change. Whether gas formation indicates a reaction depends on the circumstances and additional evidence.
A child should learn to ask what the bubble contains and what process caused it. That cannot always be inferred from appearance alone. The Secondary 1 changes of state and heating curves article explores the state-change side in greater depth.
5. Rusting is chemical change, not a mere colour change
Iron rusts when it reacts under suitable environmental conditions, commonly involving oxygen and water. The process produces iron-containing corrosion products rather than simply changing the colour of unchanged iron. That change of substance is why rusting is classified as chemical.
In a school question, a photograph of rusty iron may accompany a description of exposure conditions. A learner should not respond “chemical because it turns brown” and stop there. The more scientific answer connects the observation to formation of new substances. The colour is evidence; the identity change is the reason for the classification.
Avoid home experiments with acids or improvised corrosion accelerants. Observing an ordinary rusted bicycle part or considering a school-provided diagram is enough to support a safe discussion. At higher levels, further reaction details can be introduced when they are part of the syllabus.
6. Burning is chemical change, but smoke is not the whole story
Burning a substance under suitable conditions involves chemical reactions, commonly with oxygen, producing new substances. The products can include gases and other materials depending on what burns and how completely. The visual effects—flame, light, smoke or char—may be noticeable, but the classification rests on chemical transformation.
A student may write, “Burning paper is chemical because it cannot become paper again.” The ordinary irreversibility is not the defining reason. A more defensible explanation is that combustion and related reactions form new substances. This matters because some physical changes also cannot easily be reversed in everyday practice.
Do not demonstrate burning at home to settle the concept. A supervised school video, safe classroom discussion or standard textbook diagram provides enough evidence for a first-year explanation. We are teaching chemical reasoning, not asking students to create a fire.
7. Cutting paper is physical even if it cannot be “uncut”
Cutting a sheet of paper into tiny pieces changes size and shape. The paper material is still paper, although real materials are complex mixtures of substances. In a conventional school-level classification, cutting is a physical change because no new substance is necessarily formed by the act of cutting.
You cannot conveniently restore the original sheet without joining the pieces. Does that make cutting chemical? No. This exposes the weakness in the rule “physical changes are reversible, chemical changes are irreversible.” Ease of reversal is not a dependable defining criterion.
I like comparing cutting paper and burning paper. Both make the original sheet unusable for its former purpose. Yet cutting changes form, while burning involves chemical transformation. If a child can explain this contrast, they are moving beyond simple reversible/irreversible memorisation.
8. Dissolving: sometimes simple, sometimes more complicated
Dissolving table salt in water is commonly taught as a physical process in introductory classifications: the dissolved components remain present and a solution forms. The substances have not simply vanished. Appropriate separation can recover constituents under relevant circumstances, though detailed particle interactions are richer than the simple textbook diagram.
Do not teach “all dissolving is always physical” as a universal claim. In real chemistry, some dissolutions are accompanied by chemical reactions, and whether a process is classified as physical or chemical depends on what changes chemically. When we teach a standard saltwater example, we must keep the example’s assumptions visible.
A useful lower-secondary answer may say that dissolving salt does not, in itself, create a new chemical compound called saltwater. A more advanced course may explain hydration and ions. The next lesson should match the child’s present vocabulary rather than overloading them with chemistry they have not encountered.
9. Sugar dissolving is not sugar melting
Two events can make visible sugar crystals disappear while involving different processes. When sugar dissolves in water, it disperses into a solution. When a substance melts, the pure substance changes from solid to liquid under relevant conditions. These should not be described as identical simply because the solid crystals are no longer visible.
Ask what else is present. In the dissolving case, water is a solvent in the system. In melting, the material itself undergoes a state change. At the intended school level, the distinction can be made without writing advanced molecular equations.
Do not attempt to melt or caramelise sugar in an unsupervised exercise. Hot sugar can cause serious burns, and heating it may produce chemical transformations in addition to physical melting under real conditions. We only need clear descriptions and carefully bounded school examples.
10. Reversible does not mean physical in every case
Some chemical reactions can proceed in both directions under appropriate conditions. Meanwhile, certain physical changes may be very difficult to reverse in practice. Reversibility therefore cannot serve as the universal definition of physical versus chemical change.
An advanced example is a reversible reaction that reaches chemical equilibrium, though that is not a topic a Secondary 1 pupil needs to calculate. At the simpler level, it is enough to know that “easy to reverse” may help describe a particular familiar process, but it does not prove no new substance formed.
Teach the learner to replace the faulty rule with the better question: “Do the substances retain their chemical identities?” Once they use that criterion, their explanations become more stable when the example changes.
11. Colour change can be evidence, not final proof
A visible colour change may be associated with a chemical reaction, such as formation of a differently coloured product. But colour can also change because of mixing, optical effects, dilution, changes in lighting or other processes that do not demonstrate a new substance on their own.
A drop of dye spreading through water changes the colour distribution of the mixture. This is not, by itself, proof of a chemical reaction. A coloured indicator changing in response to the chemical environment is a different context; its explanation depends on the substances and reactions involved. The correct conclusion should reflect what the question actually tells us.
A student who writes “chemical change because the mixture became red” may need to be asked what caused the red colour and what evidence shows new chemical substances formed. The tutor should reward supported interpretation rather than automatic confidence.
12. A temperature change is not enough by itself
Chemical reactions can release or absorb energy, causing temperature changes. Physical processes such as heating, cooling, melting, evaporation and mixing can also involve energy transfer and temperature change. We cannot classify a process as chemical solely because the thermometer moved.
Consider ice warming from −5 °C to 0 °C and then melting. The reading changes before melting and may remain approximately constant during the change of state under appropriate conditions. None of this requires the formation of a different chemical substance.
If the school question states that two substances react and the mixture becomes warmer, the temperature observation may help describe the reaction’s energy change. But the wording of the investigation, the identities involved and the evidence determine the classification. A good learner reads the whole situation.
13. Gas bubbles are not an automatic chemical test
Bubbles during boiling are a physical phenomenon. Bubbles when certain reactants are combined can be evidence of gas production in a chemical reaction. A student should not claim that every bubble is carbon dioxide or that all fizzing proves a particular reaction.
Ask the learner to identify what they can observe and what they would need to know next. “Bubbles formed” is direct observation. “A gas was produced” can often be a reasonable interpretation, but identifying the gas may require a specific test or supplied context. At this age, the habit of not overclaiming is more important than memorising every qualitative gas test.
A good example question changes the context while preserving the bubbles. One case involves heating water; another describes a known reaction. The child should explain why appearance alone does not give the same scientific conclusion in both.
14. Formation of a precipitate needs careful interpretation
When suitable solutions react to form an insoluble solid, that solid can appear as a precipitate. In a school-provided reaction context, the formation of a new insoluble substance can be evidence of a chemical change. But not every cloudy liquid proves a precipitate-forming reaction.
If someone pours fine insoluble powder into water, the mixture becomes cloudy without necessarily producing a new substance. Compare that with a described chemical reaction where a solid product is formed. The observations can look superficially similar; the processes differ.
At Secondary 1, use this as a reasoning extension where appropriate, not as a demand to memorise upper-secondary precipitation equations before the course introduces them. A well-taught beginner can already distinguish the observation “a solid appeared” from the conclusion “a new solid compound formed.”
15. A change of smell may have several causes
A smell can spread because volatile substances diffuse through air, even when no new substance forms. A new smell can also arise when a reaction produces a different volatile substance. The nose reports a sensory experience; it does not directly identify the chemical mechanism.
Take perfume dispersing in a room as a familiar example of particle movement and mixing. Compare it with food undergoing chemical changes and producing new odorous substances. The situations are not scientifically identical even if both involve smell.
The key question is not simply “Did the smell change?” It is whether the evidence supports a new chemical substance being produced. Our linked diffusion guide explains how odours spread without treating diffusion as a reaction.
16. Physical processes can accompany chemical reactions
Real-world events rarely arrive with one label stamped on them. A candle may involve wax melting and vaporising as well as combustion of wax vapour. The melting and vaporising are physical changes; the combustion is chemical. It is misleading to assume the entire sequence must be classified as only one kind.
This is a more advanced everyday example, best explored through diagrams or a teacher-led explanation rather than a home flame experiment. It illustrates why process boundaries matter. If a question asks what happened to wax near the flame, it may be discussing melting. If it asks about reaction with oxygen, it is discussing combustion.
I ask students to name the specific step before assigning a category. Scientific precision often comes from dividing a complicated event into the processes that actually occur.
17. Cooking is complicated: use the correct claim
Cooking an egg involves changes to proteins, including denaturation and aggregation under heating, and is commonly taught as an example of chemical change in school contexts. But actual cooking includes physical changes too, such as heat transfer and movement of water. The classification depends on which process is being discussed.
A simplistic response “cooking is chemical because it can’t be reversed” misses the underlying transformation and again misuses irreversibility. A more useful explanation identifies changed molecular structures and properties, without pretending a Secondary 1 student must master protein chemistry.
For young learners, familiar cooking examples can stimulate curiosity. Yet we should not imply that every step in a recipe is a chemical reaction. Mixing ingredients, pouring liquids, freezing and heating can include processes with different scientific classifications.
18. What is the role of the particle model?
A particle model helps learners describe what is changing at a level too small to see directly. During a simple physical change of state for a pure substance, the chemical particles retain their identity while arrangement, spacing and movement differ. During a chemical reaction, atomic connections can change and different chemical species can form.
In introductory drawings, circles often stand for particles. That does not mean real atoms are exactly the size, colour or spacing represented. Different models may represent molecules, atoms or ions differently. The student’s job is to read the diagram key and decide what feature matters for the question.
A useful comparison shows H₂O molecules in solid and liquid arrangements, then a different diagram of a chemical reaction involving different reactant and product species. Ask the learner why one set preserves each water molecule while the other changes which atoms are bonded. Do not ask for advanced bonding diagrams if the child’s course has not introduced them.
19. Evidence versus certainty in a school practical
Suppose a teacher gives three observations: the mixture warms, a new colour appears and a gas is collected. Together with known starting materials and suitable tests, these observations may support a conclusion that a reaction occurred. But isolated signs are not universal proof in every unknown case.
I teach students to write first what was directly observed, then what the evidence suggests, then the scientific explanation warranted by the scenario. If their language jumps from a single observation to a precise product identity, ask which test justifies the jump.
This skill carries directly into later Chemistry qualitative analysis. Students who respect the difference between observation and inference will be less likely to claim that one colour change proves a substance’s identity without the required supporting evidence.
20. Work through a classification table by reasoning
Imagine four classroom descriptions: ice becomes liquid water; an iron nail develops rust; a sheet of aluminium foil is folded; a piece of wood burns under controlled conditions. Ask the learner to classify the specific process and give a reason connected to identity.
Melting ice is physical because the substance remains water. Rusting is chemical because new corrosion products form. Folding aluminium foil is physical because shape changes without a necessary change in chemical composition. Burning wood involves chemical reactions forming new products.
Now hide the classification words and ask the student to explain what changed in each case. If they can reason correctly without a memorised table, the ideas are starting to become useful. An exam may give an unfamiliar material; the reasoning should still work.
21. A two-column answer that is worth remembering
On paper, write “Physical change: identity remains” on one side and “Chemical change: new substances form” on the other. Under each, write an example and a specific piece of supporting reasoning. Do not fill the page with ten random signs because the general rules are the part that transfers.
Then add a third column labelled “Not enough information.” Some observations cannot justify a reliable classification without context. A clear liquid turning blue might be caused by a reaction, addition of dye or a change in another condition. The scientific response may require further evidence.
I like that third column because it teaches a young learner that careful uncertainty is not failure. A scientist earns confidence by connecting a claim to evidence. When the question is genuinely underdetermined, inventing an answer is not mastery.
22. A test question about crushed chalk
A student crushes a piece of chalk into a fine powder. Is this physical or chemical? In the normal school example, it is physical. The chalk material has been broken into smaller pieces but the crushing itself does not form a new chemical substance.
A weak answer says, “physical because it can be put back together.” In reality, returning the powder to the original piece is not straightforward. The strong answer refers to a change in shape or particle size without chemical transformation.
Now change the example to grinding a different solid. If no reaction is described, the same general reasoning can apply. But if the question involves conditions under which a substance decomposes or reacts, the student must read those extra details. Avoid treating a familiar verb as a substitute for scientific context.
23. A test question about a wet towel drying
When water evaporates from a wet towel, water changes from liquid to gas. That is a physical change of the water under ordinary conditions, not a new chemical product appearing. The towel is not proving a chemical reaction merely because it becomes dry.
Ask where the water went. A good learner explains that it entered the surrounding air as water vapour rather than vanished from existence. This also prepares the conservation of mass guide, where we follow matter across a defined system boundary.
Do not assume evaporation always requires reaching the boiling point. Evaporation can occur below boiling, a useful difference between two ways liquid water becomes gas. At Secondary 1, the depth of the particle explanation should match the taught course.
24. A test question about bread toasting
Toasting bread under suitable conditions involves chemical changes that generate new flavours, aromas and colours, alongside physical changes such as moisture loss. The exact mechanisms are complex, so a simple school response should avoid invented equations. It can identify that new substances form in browning reactions.
If the question asks specifically about water evaporating from bread, the relevant change may be physical. If it asks about chemical browning, chemical change is the intended focus. Both descriptions can refer to the same overall kitchen event. The question’s boundaries matter.
This is a reason I prefer teaching with familiar situations followed by precise verbs. “What is changing?” leads to a better answer than classifying the whole scene by habit.
25. A test question about an effervescent tablet
An effervescent tablet placed in water may undergo chemical reactions releasing gas, depending on its formulation. The visible fizzing is one clue, but identifying the reaction requires information about the ingredients or products. Other substances may also dissolve during the same event, which is a physical component of the process.
For a school-style written scenario, ask what observation is made, what scientific explanation is supported and whether more evidence would be needed to identify the gas. This teaches several kinds of reasoning without handling tablets or performing home chemistry.
A strong response does not say “any tablet that fizzes becomes carbon dioxide.” It uses the information given in the task. Accurate evidence-based answers remain important even when an everyday example seems obvious.
26. G1 pathway: use visible examples and short causes
For a G1 learner, start with school-taught examples and one clear reason. Show an ice cube becoming liquid and a teacher-provided image of rusting iron. The student states what can be seen, then chooses whether the material’s identity remains or a new substance forms, using the vocabulary expected in their course.
If chemical change is not yet in the student’s current unit, the lesson can begin with state changes and simple physical processes. There is no need to impose advanced chemical equations to prove seriousness. Introduce a new category only when the child has enough understanding to use it correctly.
The MOE G1 Lower Secondary Science syllabus and school lesson plan define the scope. A well-chosen two-question contrast is more valuable than an oversized worksheet whose content is out of sequence.
27. G2 pathway: explain a change using the model
For a G2 learner who is studying relevant concepts, ask not only for the label but the reason. “The water molecules remain water molecules” provides a model-based explanation for melting. “New substances form during rusting” explains why the corrosion example is different.
The tutor can present two similar observations, such as bubbles during boiling and bubbles in a stated gas-producing reaction. The child then learns to distinguish physical appearance from chemical mechanism. The right answer is not “bubbles mean chemical”; it depends on what else is known.
Use the MOE G2/G3 Lower Secondary Science syllabus as a curriculum reference, while following the child’s actual classroom order. A changed scenario is an excellent assessment of genuine understanding.
28. G3 pathway: challenge absolutes and incomplete evidence
For a G3 learner with the necessary prior teaching, I would introduce more carefully worded cases. “Every irreversible process is chemical.” Counterexample: tearing or crushing something may be impractical to reverse but not create a new substance. “Every colour change proves a reaction.” Counterexample: mixing dye into water changes appearance without establishing chemical transformation.
Next ask the student to explain why the incorrect rule was appealing. Perhaps they saw it on a mnemonic sheet. Perhaps a school example happened to fit. The important repair is to identify the true criterion and recognise its limits.
A useful extension asks whether a single real-world event can contain both physical and chemical changes. The student can reason from a written candle scenario or cooking context with suitable caveats. The purpose is depth of reasoning, not to rush into senior Chemistry calculations.
29. How to correct a common written answer
Suppose the child writes, “Rusting is chemical because it changes colour and cannot be reversed.” I would ask whether colour change is the reason or one possible observation. Then ask what the iron has reacted to produce. The improved answer focuses on formation of new substances, with relevant conditions or evidence if requested.
Now ask them to classify melting ice. The weak earlier rule would mark it physical because it reverses. The improved rule says no new chemical substance is formed, whether the ice happens to be frozen again or not. The same reasoning handles both examples consistently.
Good marking feedback identifies a specific missing connection. “Be more detailed” may encourage extra irrelevant words. “Explain whether a new substance formed” tells the child what to think about on the next attempt.
30. A ten-minute revision routine for this topic
First two minutes: with notes closed, write the definitions of physical and chemical change in your own words. Next three: classify four familiar processes and give a one-line reason. In minutes six and seven, inspect two tempting wrong statements about reversibility and bubbles. In the final three, answer a changed situation without seeing the corrected examples.
Record whether the child needed a prompt. If they did, revisit the relevant contrast later. If they can explain both an ordinary example and a changed one independently, move on. The best evidence of progress is not a perfectly copied answer but the ability to use the criterion in a new situation.
This can sit inside a 20-minute Science revision routine without adding another whole hour to the student’s evening. A short, well-targeted explanation is often enough for one specific misconception.
31. Where this leads in Secondary 3 and 4 Chemistry
Later Chemistry lessons involve symbols, chemical formulae, balanced equations, reaction conditions and deeper reasoning about products. The idea that new substances form remains foundational. So does the distinction between evidence and explanation. A student who uses these correctly in Secondary 1 has a more secure starting point for later learning.
This does not mean all students need to master upper-secondary reactions now. G1, G2 and G3 pathways and subject choices differ. What remains helpful is a habit of asking which materials are present before and after an event and whether the question is describing a physical process, a chemical reaction or both.
A tutor who prepares students for the future should not sacrifice the current syllabus to a showy list of advanced terms. Teach the present concept deeply enough that future ideas can attach to it.
32. How parents can select appropriate help in Hougang
Bring a marked Science question to any prospective tutor and ask them to diagnose why the answer failed. Does the learner confuse changed appearance with changed chemical identity? Do they misunderstand particles? Do they use the word “reversible” as a universal test? Each problem suggests a different next lesson.
In a small group, the tutor should allow each child to explain a different example and then attempt a new one independently. A classmate’s correct sentence should not be counted as another child’s understanding. If there is progress, the student should be able to explain a changed scenario without the tutor providing the key phrase.
Confirm lesson location, travel time, class size, actual subject level and availability directly. A Hougang educational page gives guidance relevant to local families; it is not a guarantee of a physical teaching slot or exam result. A clear diagnosis and a sensible plan are more informative than promotional promises.
Frequently asked questions
Is melting a chemical change because heat is added?
No. Ordinary melting of a pure substance changes its state while the chemical identity remains the same. Energy transfer can accompany physical and chemical changes.
Are all chemical changes irreversible?
No. Some chemical reactions are reversible under appropriate conditions. Reversibility is not the defining criterion.
Does a colour change always prove a reaction?
No. A colour change may be relevant evidence but can have other explanations. Interpret it alongside the substances, conditions and other observations.
Does bubbling always mean carbon dioxide formed?
No. Bubbles can arise from boiling or dissolved gases leaving a liquid, and a chemical reaction may produce different gases. Identification requires appropriate evidence or given context.
Is dissolving always a physical change?
Simple cases such as dissolving salt in water are commonly treated as physical changes in introductory Science, but real dissolving processes may be accompanied by chemical reactions. The precise situation matters.
Can one everyday process involve both physical and chemical changes?
Yes. Cooking and candle use, for example, can include state changes and chemical reactions. Identify the specific step the question asks about.
A sensible next step
If the main difficulty is knowing what substances are present, start with Elements, Compounds and Mixtures Explained. For particle motion without chemical reaction, read Diffusion in Gases and Liquids. For tracking materials through changes, use Conservation of Mass in Closed and Open Systems. And if the learner struggles to write the reason, use the Explain, Compare and Predict Science Questions guide.
For school requirements, check the MOE G1 syllabus or MOE G2/G3 syllabus. The Royal Society of Chemistry’s secondary teaching resources offer further context, while the Hougang Chemistry reference library maps the broader pathway.
If the student can tell me what changed physically, what changed chemically and what evidence supports the conclusion, I would call that a much stronger result than a memorised list of twenty examples. The idea is ready to leave the worksheet and enter the world.