Water does not stay put. The same molecule can be ice on a glacier, vapor in a cloud, rain on a roof, and a river heading to sea, then evaporate and begin again. That circulation is the water cycle. The Sun is the engine. It warms oceans, lakes, soil, and leaves. Liquid becomes vapor and rises. As air cools with height, vapor condenses on tiny particles and forms clouds. When droplets or ice crystals grow heavy, they fall as rain, snow, sleet, or hail.

What lands on the ground has several fates. Some runs off into streams. Some soaks in and becomes groundwater, moving slowly through pores and cracks, feeding wells and springs years later. Plants pull water up and release it through stomata, a process called transpiration that rivals evaporation from bare soil in many landscapes. Ice sheets and mountain glaciers store water for centuries or millennia, then release it when climate warms.

The cycle is closed in mass but not even in time or place. Deserts sit under sinking dry air. Rainforests recycle moisture so efficiently that much of their rain is local. Humans interrupt the path with dams, irrigation, and wells that empty aquifers faster than rain can refill them. Every drink is borrowed from this loop. The water in a glass may have been through a cloud, a root, and a whale, and it will go around again.
