Daily Read / Everyday Physics
Everyday Physics · Cause-and-effect chain and evidence
Why Does a Cold Drink Get Wet on the Outside?
A cold cup can become covered with water even when it is not leaking. Follow the evidence from invisible water vapour to visible drops and learn how dew point helps explain the change.
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A puddle appears, but the cup is not necessarily leaking
Set a cold drink on a table and wait. Soon, tiny beads may cover the outside. They merge into larger drops, slide down the cup and leave a wet ring. It is easy to imagine that some of the drink somehow squeezed through the wall of the cup.
The U.S. Geological Survey uses this exact everyday example when explaining condensation: water on the outside of a cold glass on a humid day is water vapour from the air turning into liquid water. The visible drops are new liquid on the outer surface, not evidence that the drink passed through the cup.[1]
The room already contains invisible water
Air can contain water in gas form, called water vapour. You usually cannot see that vapour. NOAA explains that humidity describes water vapour in the atmosphere, while relative humidity compares the amount present with the maximum possible at the current temperature.
That means the space around an apparently dry cup is not necessarily free of water. Some water is mixed through the air as an invisible gas. The cold cup changes what happens to the vapour immediately beside its surface.[2]
The cold surface cools nearby air
The drink and cup are colder than the surrounding room. Air touching the cold surface loses energy and cools. As the nearby air gets colder, it can reach a point where some water can no longer remain as vapour under those conditions.
NOAA calls the temperature at which water vapour turns into liquid droplets the dew point. If the cup surface is cold enough to cool nearby air to that condition, condensation can begin on the outside of the cup.[2]
Condensation makes the droplets
Condensation is the change from gaseous water vapour to liquid water. USGS describes it as the opposite of evaporation. On the cup, the process happens right where moist air meets the cold surface, so tiny liquid droplets collect there.
The same process appears in other familiar places. Glasses can fog when they meet warm humid air, cooler windows can collect moisture, and fog can form when moist air cools to its dew point. The settings differ, but the key pattern is cooling followed by condensation.[1]
Humidity changes how dramatic the effect looks
If the surrounding air contains more moisture, there is more water vapour available to condense. NOAA notes that a higher dew point means more moisture is present in the air. That is one reason a chilled cup may bead up quickly on a muggy day but stay much drier in very dry air.
This does not mean relative humidity alone predicts every drop on every cup. Surface temperature, air movement and moisture all matter. The useful reading move is to identify the variables the explanation actually names instead of turning one factor into a universal rule.[3]
Test the explanation with predictions
A good causal explanation should make predictions. If the water comes from the air, a sealed cold container can still collect moisture on its outside. A colder surface should generally make condensation more likely when moist air contacts it. Drier air should generally provide less vapour to condense.
Those predictions are stronger than simply saying, “The cup looks wet, so it leaked.” They connect the observation to a mechanism that can be checked in other situations. Careful readers ask not only what happened, but what evidence would look different if another explanation were true.[1]
Fast lesson
Use these checks before you answer the practice questions.
Students may identify condensation but still think the liquid came through the cup. Repeatedly ask where the water molecules were before they became droplets.
Have learners state the sequence: water vapour in air → nearby air cools → dew-point condition → condensation → visible droplets.
The goal is conceptual understanding, not calculating dew point. It marks conditions where vapour can begin turning into liquid.
Good evidence should help tell competing explanations apart, such as whether an intact sealed container can still become wet outside.
Common mistakes
Why this fails: That nickname can hide the mechanism. In ordinary condensation, the outside droplets come from water vapour in the surrounding air.
Why this fails: Water vapour is gaseous water and is normally invisible; visible fog or mist is made of tiny liquid droplets.
Why this fails: Air can contain water vapour at low temperatures; the amount and saturation conditions change with temperature.
Why this fails: Condensation depends on the moisture present and on temperatures, including the cold surface that cools nearby air.
Word Lab
Use the meaning, hear the word, then try it in your own sentence.
condensation
nounThe change of water vapour from a gas into liquid water.
Condensation formed droplets on the cold cup.
water vapour
nounWater in gas form mixed with the air.
Invisible water vapour was already in the room air.
humidity
nounA description of water vapour in the air.
High humidity means the air contains a lot of moisture.
relative humidity
nounThe amount of water vapour in air compared with the maximum possible at that temperature.
Relative humidity changes when temperature changes.
dew point
nounThe temperature at which air becomes saturated and water vapour begins to condense under the given conditions.
Cooling air to its dew point can lead to droplets.
surface
nounThe outside or top layer of an object.
Drops formed on the outer surface of the cup.
variable
nounA factor that can change and affect a result.
Air moisture is one variable that affects condensation.
prediction
nounA statement about what should happen if an explanation is correct.
The condensation explanation makes a prediction about dry air.
Check your understanding
Choose an answer, explain your choice, and then check. Hints and retries are welcome.
Try the questions here. Public practice does not save a learning record.
Foundation
Foundation practice
Trace the literal details and the basic cause-and-effect chain.
1. What form is the water in before it becomes a droplet on the cup?
2. What is condensation?
3. What does the cold cup do to the air touching it?
4. What does dew point mark in this article?
5. Visible droplets prove that an intact cup leaked.
6. Put these in order: droplets form; nearby air cools; vapour is in the air.
Evidence
Evidence practice
Separate observation, explanation and useful evidence.
7. What can you directly observe after a cold cup sits out?
8. What is the article’s explanation for where those droplets came from?
9. Why is a sealed cold container useful evidence?
10. Which is stronger: “it is wet, so it leaked” or “a sealed intact cup gets wet because air moisture condenses”?
11. Name one variable besides cup temperature that can affect how much condensation you see.
12. Would the colour of the cup be strong evidence for the source of the water?
Reasoning
Reasoning practice
Use the mechanism to make predictions and reject common misconceptions.
13. What would you generally expect in drier air, all else similar?
14. What would you generally expect if the cup surface were warmer and no longer cool enough to reach condensation conditions?
15. Why can glasses fog when someone moves into warm humid air?
16. A student says “water vapour is the white mist I can see.” Correct the statement.
17. Why is “humidity causes condensation” incomplete?
18. How is cloud formation related to the cup example?
Synthesis
Synthesis practice
Build concise evidence-based explanations from multiple parts of the text.
19. Summarize the article in one sentence.
20. Write the four-step chain that explains the wet cup.
21. What observation would make an actual leak more plausible?
22. Which source directly gives the cold-glass example?
23. Give one claim and one supporting piece of evidence from the article.
24. Why does the article compare a simple leak idea with condensation?
Practice answer key
- 1. Water vapour in the air.
The article says the surrounding air already contains invisible water vapour. - 2. Water vapour changing from gas to liquid.
Condensation is the phase change that produces the droplets. - 3. It cools the nearby air.
The surface removes energy from the air immediately beside it. - 4. The temperature condition at which water vapour begins to condense.
Dew point is used as the threshold idea for condensation. - 5. False.
Condensation can create water on the outside without a leak. - 6. Vapour is in the air → nearby air cools → droplets form.
That order follows the causal chain in the passage. - 7. Liquid droplets can form on the outside surface.
The droplets are observable; their origin requires explanation. - 8. Water vapour in the surrounding air condensed.
This conclusion uses the condensation mechanism and source evidence. - 9. It can still become wet outside even though liquid from inside cannot simply leak through an intact wall.
The observation helps distinguish condensation from leakage. - 10. The second claim is stronger.
It identifies a mechanism and can make testable predictions. - 11. Air moisture or humidity.
The amount of water vapour available affects potential condensation. - 12. No.
Cup colour does not distinguish condensation from leakage in this explanation. - 13. Less condensation on the cup.
There is less water vapour available to condense. - 14. Less or no condensation.
The nearby air may not be cooled to the dew-point condition. - 15. Moist air contacts cooler lenses, cools, and water vapour condenses into droplets.
It is the same gas-to-liquid process described for the cup. - 16. Water vapour is normally invisible; visible mist is tiny liquid droplets.
The gas itself is not the visible cloud of droplets. - 17. Temperature and cooling conditions matter too.
Moisture provides vapour, but the air near the surface also needs to cool enough for condensation. - 18. Both involve water vapour cooling and condensing into liquid droplets.
The scale and setting differ, but the phase change is the same. - 19. Water on a cold cup can form when water vapour in surrounding air cools and condenses on the surface.
A strong summary names the source, cooling and phase change. - 20. Water vapour in air → air near cold cup cools → dew-point condition is reached → liquid droplets form.
Each arrow names a causal step rather than only the beginning and end. - 21. Liquid level dropping while moisture appears from a crack or damaged spot.
That pattern would provide evidence tied to liquid escaping from inside. - 22. The U.S. Geological Survey.
USGS explicitly uses water on a cold glass as a condensation example. - 23. Claim: outside water can come from the air. Evidence: an intact sealed cold container can still collect outside droplets.
The evidence helps distinguish the claim from the leak explanation. - 24. To show how evidence and a causal mechanism can replace a tempting first explanation.
The comparison teaches both the science and a reading/reasoning habit.
Related skills
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
Water on a sealed cold bottle can come from the air, not from inside the bottle. Air contains invisible water vapour. The cold bottle cools nearby air, and if the conditions reach the dew point, some vapour condenses into liquid droplets on the surface. A sealed intact bottle can still collect those droplets, which supports condensation rather than leaking.
Sources and revision notes
Source check: 2026-09-23. Grade guidance is an editorial suggestion, not a standardised reading score.
- U.S. Geological Survey — Condensation and the Water Cycle
- NOAA NESDIS — What Is Humidity?
- National Weather Service — Dew Point vs. Humidity
What changed in this edition?
Initial release. Condensation, humidity and dew-point explanations were checked against USGS, NOAA and National Weather Service sources on 2026-09-23. Passage, examples and questions are original Study Commons material.