Be a drop of water: up from the sea, into a cloud, down as rain or snow, and back again.
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Imagine you are one tiny drop of water in the middle of the Atlantic Ocean. The sea holds about 97% of all the water on Earth. A drop can stay in the sea for thousands of years, but today you’re off on a trip.
The sun warms the sea. Your drop soaks up the heat and floats up into the air as water vapour, an invisible gas. This is called evaporation, and the salt stays behind in the sea.
Rising air carries you up high, where it is cold. You cool down and turn back into a tiny drop of liquid water. This is condensation, and lots of tiny drops together make a cloud.
Inside the cloud, tiny drops bump together and grow. When they get too heavy for the rising air to hold up, they fall as rain or snow: precipitation. A little over 9 in 10 drops that leave the sea fall straight back in, but the wind blows you over land.
Where you land changes your whole trip. Tap a path to follow your drop. The times are rough averages, so one real drop could be much quicker or slower.
Almost every path leads back to the sea in the end. A river drop is back in weeks, groundwater takes hundreds of years, and ice can hold a drop for more than 1,000 years. Then the sun can lift it up again: this is the water cycle.
Some places hold a drop of water for a few weeks. Others hold it for hundreds or thousands of years.
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The globe pictures are simplified to show the idea. Earth images: NASA Blue Marble (public domain).