The water cycle — Science, 11–13 years
Water moves between oceans, land, air and living things instead of being used once and vanishing. Heating, cooling and gravity drive this repeating journey through evaporation, condensation, precipitation and collection.
The idea
The same water can change place and state many times. Sunlight warms liquid water so it evaporates; higher, cooler air lets vapour condense into droplets; gravity brings rain or snow down. Water then collects in soil, rivers, lakes and seas, ready to move again.
Why the cycle matters
People once saw rain, rivers and clouds as separate events, but needed to understand where usable water comes from and why it can run short. The cycle links them into one system. It helps explain floods, droughts and why protecting water sources matters.
A puddle after rain
After rain, a puddle lies on warm pavement. First, sunlight warms it and some water evaporates into invisible vapour. The vapour rises and cools; in a cloud it condenses into droplets. If droplets grow heavy enough, precipitation falls and may refill the puddle or flow elsewhere.
The common mistake
A reasonable mistake is thinking evaporation happens only when water boils, or that clouds are made of smoke. Evaporation can happen slowly from a cool puddle, and clouds contain tiny liquid droplets or ice crystals. Boiling is rapid evaporation throughout the liquid, not the only kind.
Where it appears
Weather forecasts use the cycle to predict rain, snow and fog. Reservoirs and water-treatment systems depend on water moving through catchments, while farmers plan around rainfall and soil storage. The cycle does not make every place equally supplied: clean water can still be local and limited.
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