Rain in a Bag: Building a Mini Water Cycle
Where sunlight, evaporation, and condensation replay Earth’s water story
Time: 15 minutes to set up, a few hours to observe. Complexity: Beginner Setup: Ziplock bag, water, tape, sunny window
Overview 
Every drop of rain that has ever fallen has been recycled through the same endless loop for billions of years. In this water cycle activity, you will trap that entire loop inside a single sealed sandwich bag, tape it to a sunny window, and watch evaporation, condensation, and rainfall all happen in miniature over the course of a single afternoon.
This is one of the simplest ways to see the water cycle as a continuous, self-powered system rather than just a diagram in a textbook, and it needs almost nothing beyond a bag, some water, and a sunny spot.
A sealed bag taped to a sunny window, with blue water below and droplets forming above.
Materials Required
| A clear Ziplock bag
sandwich or quart size |
Water
a few tablespoons |
| Blue food coloring
to represent the ocean |
Clear tape
to mount it on glass |
| A sunny window
the sunnier the better |
A permanent marker (optional)
to label the stages |
Step-by-Step Instructions
| 1 | Add colored water: Pour a few spoonful’s of blue-tinted water into the bottom of the bag. |
| 2 | Seal the bag: Press out excess air and zip the bag shut firmly so no water can leak. |
| 3 | Tape it to a window: Fix the bag flat against a sunny window using strips of tape along the top. |
| 4 | Wait and watch: Check back every hour or two, looking closely at the upper half of the bag. |
| 5 | Record what changes: Note when droplets first appear and when they begin sliding back down. |
A sealed bag becomes a tiny self-contained water cycle.
| Pro Tip for water cycle Activity
Choose the sunniest window available and set up the bag in the morning — the more direct heat it receives, the faster and more dramatic the evaporation and condensation cycle will be. |
The Science Behind the water cycle Activity
Sunlight warms the water at the bottom of the bag. As it warms, molecules gain enough energy to escape as invisible vapour — evaporation, exactly as it happens over oceans and lakes outside, just faster and smaller.
That vapour reaches the upper bag, which is slightly cooler since it is further from the sun-warmed water. Cooler air cannot hold as much vapour, so it condenses back into droplets clinging to the plastic — the same process that forms clouds. Once droplets gather enough, gravity pulls them down in tiny streams that mimic rainfall, rejoining the water below and starting the cycle again.
Evaporation carries water upward, condensation gathers it into droplets, and gravity brings it back down as ‘rain’.
Expected Results
Within the first hour or two of strong sunlight, expect fine misting on the inside of the upper bag. By hour three or four, that mist typically gathers into droplets, some heavy enough to trickle back down like tiny rivulets of rain, joining the pool of water at the bottom.
Try a smaller snack bag, a jar terrarium, or comparing a sunny window to a shaded one.
Fun Variations to Try
- Use a small snack-sized bag and compare its speed to a larger one.
- Build a jar terrarium with soil and a plant for a living version of the cycle.
- Tape one bag in full sun and another in shade to compare results side by side.
- Label the stages directly on the bag with a marker as they happen.
What Did You Learn?
Evaporation needs energy. Sunlight warms water enough for molecules to escape as invisible vapour.
Condensation needs cooler air. Vapour turns back into visible droplets once it reaches a cooler surface.
Gravity drives ‘rainfall’. Once droplets grow heavy enough, they slide back down, mimicking real rain.
The cycle is continuous. A sealed system lets the same water evaporate and condense again and again.
Scale does not change the science. The same three stages power Earth’s entire global water cycle.
Conclusion
With a sandwich bag, a little coloured water, and a sunny window, you captured the entire water cycle in miniature — the same evaporation, condensation, and rainfall that has been quietly recycling Earth’s water for billions of years, now visible in the palm of your hand.
Frequently Asked Questions
Why does the bag need to be sealed so tightly?
A sealed bag traps the same water molecules inside permanently, letting them evaporate and condense repeatedly instead of simply drying out and escaping into the room.
Why does condensation form only near the top of the bag?
The upper part of the bag sits slightly further from the direct warmth of the water and sunlight, making it just cool enough for vapour to condense there.
Does this really represent the whole water cycle?
Yes — it captures the three core stages of evaporation, condensation, and precipitation, though it skips large-scale stages like ocean currents and groundwater flow.
Why do some bags work faster than others?
Stronger, more direct sunlight and a bigger temperature difference between the water and the upper bag both speed up the cycle noticeably.
