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Everyday Physics · Reasoning and evidence quality

Where Did the Puddle Go?

A shrinking puddle becomes an evidence lesson: learn what evaporation is, which conditions can change its rate, and how a fair comparison can separate a plausible explanation from a weak guess.

Suggested Grades 4–6623 core-passage words27 questionsUpdated 2026-09-25
What you’ll practiseJudge whether evidence actually tests a claim, identify variables that make a comparison fair or unfair, and explain why repeated measurements are stronger than a single observation.

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How to test an evaporation explanation: observe the change, control variables, compare one condition and repeat measurements. Test the explanation, not just the observation 1 · OBSERVE puddle gets smaller 2 · FAIR TEST same starting watersame container + time change one conditioncompare what remains 3 · REPEAT measure againlook for the same pattern stronger evidence Evaporation: liquid water → water vapour One shrinking puddle is an observation, not a complete causal test.
A shrinking puddle is an observation; fair comparisons and repeated measurements provide stronger evidence about what changes evaporation rate.

The puddle that seemed to vanish

After morning rain, a shallow puddle sat beside a schoolyard fence. By lunch it was smaller. By the end of the afternoon, most of the visible water was gone even though nobody had swept it away. The direct observation is simple: the amount of liquid water on the pavement decreased. The explanation requires more reasoning.

Water does not need to boil in order to enter the air. Evaporation changes liquid water into water vapour, an invisible gas. The U.S. Geological Survey describes evaporation as a major way water moves from Earth’s surface into the atmosphere. That makes evaporation a strong candidate explanation for the shrinking puddle, but a candidate is not the same as proof of every detail.[1]

Energy helps molecules escape the liquid

Liquid water contains moving molecules. Energy from the Sun and the surroundings can help some molecules at the surface separate from the liquid and enter the air as water vapour. USGS explains that solar energy drives evaporation in the water cycle. Water can therefore evaporate on an ordinary day far below its boiling point.

A warmer surface can often speed evaporation because more energy is available, but temperature is not the only variable. NOAA educational material also notes that evaporation increases with wind speed. Moving air can carry water vapour away from the surface and replace it with other air.[2]

Humidity changes the air around the water

Air already contains water vapour. NOAA defines humidity as a measure of water vapour in the air and explains relative humidity as the amount present compared with the maximum possible at that temperature. Air near saturation already contains a large amount of water vapour for its temperature.

That matters when comparing puddles. A warm, dry, breezy afternoon and a cool, humid, still morning differ in several ways at once. If one puddle disappears faster, the observation alone does not tell us which variable mattered most. Strong evidence requires a comparison designed around the claim being tested.[3]

A fair test changes one main thing

Suppose a class wants to test the claim that moving air can increase evaporation. They could place equal amounts of water in identical shallow trays at the same time and location, keeping both at the same temperature as closely as practical. A fan could move air over one tray while the other remains out of the fan’s airflow. Measuring the water level or mass at regular intervals would produce evidence that directly addresses the airflow claim.

The comparison becomes weaker if one tray begins with more water, one is wider, or one sits in sunlight while the other sits in shade. Those extra differences are confounding variables: they provide competing explanations for any difference in the result.[4]

One result is useful; repeated results are stronger

Even a careful test can be affected by small measurement errors or changing room conditions. Repeating the comparison helps show whether the same pattern appears again. Several consistent trials are stronger evidence than one dramatic result because they make chance or a one-time mistake less likely to control the conclusion.

Repeated evidence still needs careful language. If the moving-air tray loses water faster in repeated fair trials, the evidence supports the claim that increased airflow can increase evaporation under those test conditions. It does not prove that wind is the only factor controlling every puddle outdoors.[4]

Match the evidence to the size of the claim

Good reasoning keeps the claim no bigger than the evidence. A photograph of one dry patch can show that water is no longer visible there, but it cannot by itself measure the amount that evaporated or rule out drainage. A series of measurements from matched trays can test a specific factor more directly.

The disappearing puddle is therefore both a water-cycle story and a reading skill. Ask: What was observed? What explanation is being proposed? What other explanations are possible? What measurement would distinguish them? When evidence and claim match, the explanation becomes much stronger.[1]

Fast lesson

Use these checks before you answer the practice questions.

Start with what is observed

“The puddle is smaller” is an observation. “The sun caused it” is a causal explanation that needs a comparison or measurement.

Match the test to the claim

To test airflow, change airflow while keeping other major conditions as similar as practical.

Watch for confounding variables

Different starting volumes, container shapes, sunlight or temperatures can create competing explanations.

Repeat before generalizing

Consistent repeated trials strengthen a pattern, but conclusions should still be limited to what the design actually tested.

Common mistakes

“The puddle disappeared after the Sun came out, so sunlight was the only cause.”

Why this fails: Timing alone does not isolate a cause; other conditions may have changed too.

Comparing a wide tray in sunlight with a narrow cup in shade to test airflow.

Why this fails: Too many variables changed at once, so the result cannot be assigned confidently to airflow.

Calling water vapour a visible cloud of tiny liquid drops.

Why this fails: Water vapour is invisible gas; visible mist or cloud contains tiny liquid droplets or ice particles.

Assuming one successful trial proves a rule for every puddle and climate.

Why this fails: A single trial has limited scope. Repetition and varied conditions are needed before broader conclusions.

Using a measurement that does not address the claim.

Why this fails: Good evidence must be relevant. A photo of the tray colour does not test evaporation rate.

Word Lab

Use the meaning, hear the word, then try it in your own sentence.

evaporation

noun

The change of liquid water into water vapour.

Evaporation made the shallow water gradually disappear.

water vapour

noun

Water in its invisible gas form.

Evaporated water entered the air as water vapour.

humidity

noun

The amount of water vapour in the air.

The weather report showed high humidity.

variable

noun

A factor that can change in an investigation.

Airflow was the variable the class wanted to test.

control

verb / noun

To keep a factor the same, or a comparison condition used to judge the effect of a change.

The class tried to control the starting amount of water.

confounding

adjective

Making a result harder to interpret because another factor changed too.

Different tray sizes would create a confounding variable.

trial

noun

One run of a test or investigation.

The class repeated the trial three times.

evidence

noun

Information used to support or challenge a claim.

Repeated measurements provided stronger evidence than a single glance.

Check your understanding

Choose an answer, explain your choice, and then check. Hints and retries are welcome.

Observation vs explanation1. Which statement is a direct observation rather than an explanation?
Show answer and explanation

Answer: The puddle became smaller during the day.. The shrinking puddle is observable. The other choices make causal claims that require evidence.

Science fact2. What is evaporation?
Show answer and explanation

Answer: Liquid water changing into water vapour.. Evaporation changes liquid water into water vapour.

Evidence quality3. Which comparison best tests whether moving air affects evaporation?
Show answer and explanation

Answer: Equal water in identical trays, with a fan over one tray and other conditions kept as similar as possible.. The matched-tray comparison focuses on airflow and reduces competing explanations.

Try the questions here. Public practice does not save a learning record.

Foundation

Foundation practice

Secure the water-cycle facts and key evidence vocabulary.

Recall

1. What state change happens during evaporation?

Recall

2. Does water have to boil to evaporate?

Recall

3. Name one energy source that drives evaporation in the water cycle.

Recall

4. According to NOAA educational material, what can increased wind speed do to evaporation?

Vocabulary

5. What is humidity?

Vocabulary

6. What is a trial?

Evidence Match

Evidence Match practice

Decide whether evidence directly tests the claim.

Evidence match

7. Claim: moving air increases evaporation. Which evidence is stronger: a matched fan/no-fan tray comparison or a photo of one dry sidewalk?

Evidence match

8. Claim: one tray lost more water. What measurement would directly support it?

Relevance

9. Does the colour of a tray directly test how fast water evaporates?

Scope

10. If a fan tray loses water faster in three fair trials, what claim is justified?

Scope

11. Does that result prove wind is the only factor controlling every outdoor puddle?

Observation vs claim

12. “The water level dropped 4 mm” is what kind of statement?

Fair Test

Fair Test practice

Spot controls, confounding variables and better experimental designs.

Design

13. To test airflow, why should the trays start with the same amount of water?

Design

14. Why use identical tray shapes?

Confound

15. One tray is in direct Sun and one is in shade while only one also has a fan. What is wrong with the test?

Improve

16. A class measures only the final water level. What extra measurement would improve the comparison?

Repeat

17. Why repeat the same comparison several times?

Control

18. If room temperature changes sharply halfway through only one trial, how should that result be treated?

Reasoning

Reasoning practice

Calibrate conclusions and connect observations to explanations.

Reasoning

19. A puddle shrinks on a cloudy day. Is evaporation still possible?

Competing explanations

20. Besides evaporation, name one reason visible water on pavement might decrease.

Calibrated language

21. Rewrite “The fan proves wind always dries puddles” more carefully.

Evidence quality

22. Which is stronger for an evaporation-rate claim: one end photo or measurements every 10 minutes?

Synthesis

23. Give the chain from liquid puddle to atmospheric water.

Summary

24. Summarize the article in one sentence.

Practice answer key

  1. 1. Liquid water changes into water vapour.
    Evaporation moves water from liquid to gas.
  2. 2. No.
    Evaporation happens at ordinary temperatures; boiling is a different, faster process throughout the liquid.
  3. 3. The Sun.
    Solar energy helps water molecules leave the liquid surface.
  4. 4. It can increase evaporation.
    Moving air can carry water vapour away from the surface.
  5. 5. A measure of water vapour in the air.
    NOAA uses humidity to describe the amount of water vapour present.
  6. 6. One run of a test or investigation.
    Repeating trials lets you check whether a pattern is consistent.
  7. 7. The matched fan/no-fan tray comparison.
    It changes airflow while keeping other major factors similar.
  8. 8. The change in water mass, volume or level measured for both trays over the same interval.
    A quantitative before-and-after measure directly addresses water loss.
  9. 9. No.
    Tray colour may matter only if it changes temperature; colour alone is not an evaporation-rate measurement.
  10. 10. Under those test conditions, increased airflow was associated with faster evaporation.
    The wording matches the conditions actually tested.
  11. 11. No.
    Outdoor puddles also differ in temperature, humidity, drainage, surface area and other conditions.
  12. 12. A measurement/observation.
    It reports a measured change without explaining its cause.
  13. 13. So starting volume does not become a competing explanation.
    A fair comparison keeps major non-tested variables the same.
  14. 14. Surface area and depth can affect evaporation, so identical shapes reduce that confounding difference.
    The goal is to make airflow the main changed variable.
  15. 15. Both airflow and heating changed.
    You could not tell which difference caused the result.
  16. 16. Record the starting level too, or measure change from start to finish.
    A change requires both a baseline and an endpoint.
  17. 17. To see whether the pattern is consistent and reduce the influence of one-time error.
    Replication strengthens evidence.
  18. 18. With caution, because temperature became another changing variable.
    The trial no longer isolates airflow as cleanly.
  19. 19. Yes.
    Evaporation occurs below boiling and does not require direct sunshine, although conditions affect its rate.
  20. 20. It could drain, soak into a porous surface, flow away, or be displaced.
    Competing explanations should be considered before assigning one cause.
  21. 21. In this matched-tray test, moving air increased the rate of water loss, supporting airflow as one factor that can increase evaporation.
    The revision matches the scope of the evidence.
  22. 22. Repeated measurements over time.
    A time series shows the rate and pattern of change rather than only one endpoint.
  23. 23. Energy reaches liquid water → some surface molecules escape → liquid water becomes invisible water vapour in the air.
    That is the central evaporation process described by the sources.
  24. 24. A shrinking puddle is explained by evaporation, but strong reasoning uses fair comparisons, relevant measurements and repeated trials to decide which conditions affect the rate.
    The summary includes both the science and the reading skill.

Related skills

Why Does a Cold Drink Get Wet on the Outside?Why Does Metal Feel Colder Than Wood?Why Do Lakes Freeze From the Top?

Put it in your own words

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Check your own explanation

This is self-review, not automatic marking.

Compare with a model response

The observation is that the puddle became smaller during the day. Wind may have increased evaporation, but temperature, humidity, drainage and the pavement surface could also affect the amount of visible water. A stronger test would use equal water in identical trays and expose one tray to moving air while keeping the other major conditions as similar as possible. Measuring the change in mass or water level over the same time interval would directly compare water loss. Repeated trials showing faster loss with moving air would support airflow as one factor that can increase evaporation under those conditions, not as the only cause of every disappearing puddle.

Sources and revision notes

Source check: 2026-09-25. Grade guidance is an editorial suggestion, not a standardised reading score.

  1. U.S. Geological Survey — Water Cycle
  2. U.S. Geological Survey — The Sun and the Water Cycle
  3. NOAA NESDIS — What Is Humidity?
  4. NOAA Science On a Sphere — Water Cycle: A Water Falls Docent Script
What changed in this edition?

Initial release. Evaporation, solar energy, humidity and the role of wind in evaporation were checked against U.S. Geological Survey and NOAA educational sources on 2026-09-25. Experimental examples are original Study Commons teaching scenarios and do not claim measured results.

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