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The water cycle Water moves between the oceans, atmosphere and continents. The Sun provides the energy needed for evaporation; gravity drives rainfall and flow towards lower ground. Heat turns some liquid water in seas, lakes and soil into invisible vapour. Plants also release vapour through their leaves: this is transpiration. When moist air cools, vapour condenses into tiny droplets or forms ice crystals. Their accumulation makes clouds visible. A cloud is therefore not made of visible vapour. Droplets combine and grow. When they become heavy enough, they fall as rain, snow or hail, depending on conditions. Not all water returns immediately to the sea. On permeable ground, some rain infiltrates the soil and may reach groundwater. On a slope or impermeable ground, it runs off into streams and rivers. Groundwater can also feed a spring or watercourse. Water stays for different lengths of time in oceans, lakes, glaciers and groundwater. The cycle does not follow a single loop: a drop can take several paths and remain in a reservoir for a long time. To observe evaporation, place two dishes containing the same amount of water, one in sunlight and the other in shade. Mark the initial level and compare after a few hours. To see condensation, observe droplets on the outside of a cold glass: they come from moisture in the air. Remember: water changes state and location. The Sun and gravity keep it moving. Evaporation, condensation, precipitation, infiltration and runoff connect water reservoirs.