Table of Contents
Water Cycle in a Bag Experiment: Rain on Your Window
A water cycle in a bag experiment with two controls that make it real science, honest limits on the analogy, and the measurement that proves nothing is lost.
The part worth setting up properly is not the bag. It is the second bag. Tape one sealed bag of coloured water to a sunny window and another to a shaded wall, and by the afternoon one is covered in droplets and the other is nearly clear. Then add a third bag with its zip left open, and in three days its water level has dropped while the sealed ones have not moved a millimetre. That turns a water cycle in a bag experiment from a decoration into something a seven-year-old can reason about: energy drives it, and a closed system does not lose anything.
Key Takeaways
- You’ll tape a sealed bag of coloured water to a sunny window, with one shaded control and one open-topped control, and track the water level for three days.
- It teaches evaporation, condensation, and that a closed system conserves its water while an open one loses it.
- Best fit is ages 5 to 10, with the level-marking and the controls pitched at the older end.
- The usual failure is a bag that leaks or falls off. Use strong tape on all four edges and a bag with a working zip.
- The analogy has real limits, and saying so is part of the lesson. Those limits are listed below.
What You’ll Need
| Item | How much | Where to get it | If you don’t have it |
|---|---|---|---|
| Zip-seal sandwich bags, 1 litre | 3 | Kitchen | Freezer bags; the zip must actually seal |
| Water | 150 ml total, 50 ml per bag | Tap | Nothing else needed |
| Blue food colouring | 2 drops per bag | Baking cupboard | Washable marker ink squeezed into the water |
| Permanent marker | 1 | Drawer | A thin marker for drawing and for level marks |
| Strong tape | 1 roll | Drawer | Masking tape fails in sunlight after a day; use parcel tape |
| A sunny window | 1 | Preferably south-facing in the northern hemisphere | Any window that gets direct sun for a few hours |
| Ruler | 1 | School supplies | Marks on the bag are enough |
| Notebook | 1 | Anywhere | A photo each day on a phone |
Time: prep 15 min, first visible result 2–4 hours, full result 3 days Cost: everything is probably in your kitchen Mess level: low, unless a bag falls
Do the Experiment: The Water Cycle in a Bag Experiment
Step 1 — Draw the picture on each bag, then fill them
With the permanent marker, draw a sun in the top corner, a cloud in the middle and a wavy sea line about a third of the way up. Add 50 ml of water with two drops of blue colouring to each bag. Seal two of them completely; leave the third’s zip open.
What you should see: a small pool of blue sitting below your drawn sea line and a lot of trapped air above it. Do not squeeze the air out; the air space is where the whole process happens.
Step 2 — Mark the starting water level on all three
Lay each bag flat and draw a careful horizontal line at the water surface. Label the bags A for sunny sealed, B for shaded sealed and C for open.
What you should see: three identical bags with three identical lines. This two-minute step is what lets you claim anything at the end, and it is the step people skip.
Step 3 — Tape A to the sunny window and B somewhere shaded
Tape along all four edges of each bag so it cannot peel off, with the pool of water at the bottom. Put bag C flat on a shelf indoors, zip open and upright so nothing spills.
What you should see: nothing yet. Check back in an hour.
Step 4 — Compare A and B after two to four hours of sun
Look at the inside surface of each bag, especially the upper half.
What you should see: bag A has a fine mist of droplets across its upper inside surface, and the larger ones have started to slide down. Bag B has far fewer, often almost none. Same water, same bag, same air. The only difference is the energy arriving, which is the point of having bag B at all.
Step 5 — Check all three levels on day one and day three
Compare each water surface against its marked line, in the evening of day one and again on day three.
What you should see: bags A and B are still on their lines, within a millimetre. Bag C has dropped visibly, often by a quarter or more of its water over three days. Nothing in A was lost even though water spent all day travelling up and coming back; C lost water because the vapour had somewhere to go.
What’s Actually Going On
Liquid water molecules are in constant motion, and at any temperature a fraction of those at the surface happen to be moving fast enough to break free into the air. That is evaporation, and it costs energy: around 2,450 kilojoules per kilogram at room temperature, which is the latent heat of vaporisation, listed against temperature in the Engineering ToolBox tables. Sunlight through your window supplies that energy, which is why bag A works and bag B barely does. The USGS Water Science School’s account of evaporation covers the same step at planetary scale.
The air in the bag can only hold so much water vapour at a given temperature. Once it is saturated, any molecule that reaches a surface slightly cooler than the air condenses back into liquid, which is what the USGS page on condensation describes. The plastic on the shaded side of your bag is that cooler surface. Droplets nucleate on it, merge with neighbours as they grow, and once they are heavy enough gravity wins and they run down into the pool. That is why the level comes back to the line: in a sealed bag, every molecule that leaves has nowhere to go but back.
Now the honest part, because this analogy is usually oversold. Three things in the bag are genuinely like the real water cycle: evaporation driven by solar energy, condensation on a cooler surface, and return by gravity. The bag also models the one feature of the global cycle that children find hardest to believe, which is that it is closed. Earth gains and loses almost no water; the USGS water cycle diagrams show a budget that balances, and the overwhelming majority of that water is in the ocean at any moment.
Three things are not like the real cycle. Real clouds form when rising air expands and cools, not by touching a cold window, and they need airborne particles to condense onto, as NOAA’s explanation of cloud formation sets out. Real raindrops are not condensation sliding down a surface; they form by tiny droplets colliding and merging, or by ice crystals growing at the expense of droplets. And the real cycle has transpiration from plants, runoff, rivers, groundwater and ice storage, none of which your bag contains.
One correction to make out loud. The sun does not pull or lift water. Nothing lifts it. Individual molecules escape the surface on their own and then wander through the air, and the net effect looks like a flow upward only because there is more water below than above.
How to Explain It to Your Kid
Ages 5–8: The water goes for a walk and comes home
The sun gives the water enough energy to turn into invisible air-water and float around inside the bag. When it touches the cool plastic it turns back into drops you can see, and when the drops get fat they slide home. Point at the line you drew: the water went away and came back, and there is exactly as much as before.
Then let them compare bag A and bag B themselves, and ask which one got more sunshine. At this age the comparison carries the explanation.
Ages 9–12: Introduce evaporation, condensation and the closed system
Give the two words and use their own bags as the evidence for each. Then make the big point with bag C: the open bag lost water, and the sealed ones did not, which means the sealed bag is a closed system. The Earth is closer to the sealed bag than to the open one, which is why the water in a glass today has been round the cycle an unimaginable number of times. Our write-up of making a cloud in a jar shows the part the bag cannot do.
Ages 13+: List the analogy’s failures, then measure one
Ask them to write down three ways the bag is like the water cycle and three ways it is not, before reading the section above. That exercise is worth more than the demonstration. Then measure something real: set up four sealed bags with 50 ml each, place them in direct sun, indirect light, shade and a fridge, and photograph the droplet coverage of each at two hours. Estimate the covered fraction with a transparent grid overlaid on the photos. You have made a crude measurement of how evaporation rate depends on energy input. Pair it with bagging a leaf to prove plants lose water, which adds the transpiration the bag is missing.
The question to ask: “If we put a sealed bag in the freezer and then brought it out, where would the water be, and would the level still be on the line?”
Why Teaching This Early Pays Off
The water cycle appears in almost every primary curriculum and is almost always taught as a diagram to be labelled. Labelling a diagram teaches the vocabulary and not the mechanism, which is why so many children leave it believing that clouds are made of steam, that the sun lifts water, or that water can be used up. The bag addresses the third one directly, with a line drawn on plastic.
The closed-system idea also has a long reach beyond weather. Conservation of matter is the single most reused principle in chemistry and biology, and a child who has watched water leave and return in a sealed bag has a physical instance of it. It also sets up the honest, uncomfortable version of water scarcity: the planet is not running out of water, it is running short of fresh water in particular places at particular times, which is a different and harder problem.
Keep the claims modest. Misconceptions about where water goes are not random errors; Smith, diSessa and Roschelle argued in 1994 that they are sensible generalisations from partial experience, which is exactly what “the puddle disappeared, so the water is gone” is. Replacing that takes repeated encounters, not one bag. The National Research Council’s framework names developing and using models as one of eight core practices, and comparing a model with the thing it models, failures included, is precisely that practice done well.
Before You Start: What Parents Should Know
| Best age | 5 to 10. Fives and sixes do the drawing and the comparison; nines and tens handle the level marking and bag C. |
| Supervision | Minimal. An adult should do the taping, and should choose a window out of reach of a toddler. |
| Real hazards | None beyond normal care. A falling bag of coloured water will stain carpet and grout. Keep it away from sockets and electronics. Plastic bags are a suffocation hazard for infants, so do this well out of reach of a baby. |
| Don’t | Don’t place the bag against a halogen lamp or on a radiator to speed it up; it can soften and split. Don’t fill the bag more than a third full, or the pool rises into the condensation zone and you cannot see the droplets. Don’t drink the water afterwards. |
When It Doesn’t Work
- No droplets after a full day in sun. The bag is not actually sealed, or the window gets less direct sun than you think. Check the zip by holding the bag upside down over the sink, and check the window’s sun hours by looking at it at three different times.
- Droplets appear on the outside instead. Condensation on the room side of a cold window, which is a different effect. Dry the outside and watch for droplets forming on the inside surface only.
- The bag falls off in the afternoon. Masking tape fails in heat and sunlight. Use parcel tape or duct tape along all four edges, not just the top.
- Bag A and bag B look the same. Your shaded bag is getting indirect sunlight or sitting on a warm surface. Move it to a genuinely cool, dark place, ideally a different room.
- Bag C has not lost any water either. The zip is partly closed, or the air in your house is very humid. Open it fully and give it five days, or run it next to a dehumidifier.
Take It Further
Energy against droplet coverage. Four sealed bags, four light conditions, photograph each at two hours from a fixed distance, and estimate droplet coverage with a printed transparent grid. Plot coverage against your rough estimate of light intensity. The ordering will be consistent and the numbers are genuinely yours.
Make it rain on command. Hold an ice cube against the top of bag A for two minutes. The cooled patch condenses heavily and the droplets run down in a visible stream. Then try the same on bag B, which has less vapour available, and see how much weaker it is. That comparison separates the two halves of the cycle.
What to Watch For Over the Next 3 Months
- Week 4: they explain the condensation on a bathroom mirror or a cold drink using the word condensation rather than sweating.
- Month 2: the closed-system idea appears somewhere unexpected, often a question about whether water can run out.
- Month 3: they point out a way the bag was unlike real weather, which means the limits section landed. Encourage it; that is the most scientific thing in this article.
Frequently Asked Questions
How long until we see something?
Two to four hours of direct sun for the first visible mist, and a full day for droplets large enough to run down. A cloudy day can stretch that to two days. Set it up in the morning and check after lunch.
Does it work on a cold or cloudy day?
Yes, but slowly, and the contrast between your sunny and shaded bags shrinks. Cold glass actually helps the condensation half while hurting the evaporation half. If the weather is poor, run it for a week rather than three days.
Is this really how rain forms?
No, and that is worth saying plainly. The droplets on your bag are condensation on a cool surface, helped down by gravity. Real raindrops form in clouds when small droplets collide and merge, or when ice crystals grow and then melt on the way down. The bag gets evaporation and condensation right and precipitation wrong.
Why does the water level come back exactly?
Because the bag is sealed. Every molecule that evaporates stays inside the bag, and the only place it can end up is back in the pool. That is what a closed system means, and it is the single most useful thing the bag demonstrates.
About the author
Ricky Flores is the founder of HiWave Makers and an electrical engineer with 15+ years of experience building consumer technology at Apple, Samsung, and Texas Instruments. He writes about how kids learn to build, think, and create in a tech-saturated world. Read more at hiwavemakers.com.
Sources
- U.S. Geological Survey. “Water Cycle Diagrams.” Water Science School. https://www.usgs.gov/water-science-school/science/water-cycle-diagrams
- U.S. Geological Survey. “Evaporation and the Water Cycle.” Water Science School. https://www.usgs.gov/water-science-school/science/evaporation-and-water-cycle
- U.S. Geological Survey. “Condensation and the Water Cycle.” Water Science School. https://www.usgs.gov/special-topics/water-science-school/science/condensation-and-water-cycle
- National Oceanic and Atmospheric Administration, National Weather Service. “How Clouds Form.” JetStream. https://www.noaa.gov/jetstream/clouds/how-clouds-form
- NASA. “Water Cycle.” https://www.nasa.gov/earth/water-cycle/
- The Engineering ToolBox. “Water Properties: Vaporization Heat vs. Temperature.” https://www.engineeringtoolbox.com/water-properties-d_1573.html
- Smith, J. P., diSessa, A. A., & Roschelle, J. (1994). “Misconceptions reconceived: A constructivist analysis of knowledge in transition.” Journal of the Learning Sciences, 3(2), 115–163. doi:10.1207/s15327809jls0302_1
- National Research Council. (2012). A Framework for K-12 Science Education. National Academies Press. doi:10.17226/13165
