The Water Cycle for Kids: A Window Bag Experiment and a Jar That Keeps Itself Alive

Textbook diagrams of the water cycle show arrows going up and down, but it doesn’t mean much until students see water move. Two simple activities make it visible.

Activity 1: the window bag

You need: a clear zip-top bag, water, blue food colouring, a permanent marker and tape.

Add a few centimetres of water and a drop of blue colouring to the bag. Seal it.
Mark the water level with the marker.
Tape the bag to a sunny window. Tape a second bag, made the same way, to a shady wall.
Check both every few hours and again the next day.

What you see: In the sunny bag, clear droplets form on the upper inside of the bag, and run back down. The droplets are clear, not blue, which shows something important: the colouring stayed behind.

What’s happening

The Sun’s heat makes water evaporate: it turns into invisible water vapour and rises. At the cooler top of the bag the vapour condenses back into droplets. The droplets collect and run down, like precipitation. The colouring doesn’t evaporate, so the droplets are clear. That’s also true for the ocean and rain: salt stays behind and rain is fresh.

The shady bag changes much less, which is a fair-test comparison: more heat, more evaporation.

You can also catch the cycle in action with a cold glass of water: water droplets form on the outside, because water vapour in the room air cooled against the glass.

Activity 2: a world in a jar

Build a small ecosystem in a clear jar with a lid: pebbles, damp potting mix and a few small shade-loving plants such as moss or a fern. Mist with water, seal, and set the jar in bright light but not direct sun. Build an identical jar and leave it open as a comparison.

Over four weeks, students log what they see. In the sealed jar, water evaporates from the soil and the plants, condenses on the glass and drips back, so the jar waters itself. The plants use light, carbon dioxide and water to make food and release oxygen, which the plants and soil microbes use again. The jar is closed to matter but not to energy: it still needs light from outside.

Not every jar thrives: too much water or too little light causes mould or wilting, and that’s part of the data.

Common mix-ups

“Clouds are made of smoke or steam.” They are tiny liquid droplets.
“Rain comes from the clouds, and the water starts there.” Clouds hold water that came from oceans, lakes, soil and plants.
“The water cycle starts at the ocean.” It has no start.

Go further

Both activities are in Our Living Planet, as Where does rain come from? and Can a jar keep itself alive? Younger children can try How does a cloud form? Sign up for three lessons free.