Water cycle in a bag: evaporation on the window

Materials and 5-minute setup

You need a clear zip bag, half a cup of warm water, blue food coloring, strong tape and a sunny window. Pour the water into the bag, squeeze out most air, seal it firmly and tape all edges flat to the glass.

Choose a window with at least 4 hours of direct sun and tape the bag at child eye level. The sun will heat the water quickly, and within 30 minutes you should see tiny droplets forming on the inner plastic surface.

Evaporation, condensation and rain

Water vapor rises as invisible gas through evaporation, then cools on the plastic and turns back into droplets by condensation. When droplets grow heavy they slide down like precipitation and the small cycle starts again inside the bag.

Real clouds use exactly the same physics on a huge scale over oceans and lakes. Warm air lifts moisture, cold air squeezes it into clouds, and gravity pulls it down as rain that fills rivers and groundwater every day.

• Use warm water for faster results
• Squeeze air out before sealing
• Best sun is 10 to 3
• Droplets sliding down means rain
• Same physics makes real clouds