The water cycle

Whoo. We should get out of here! Before the lake floods... Hmm. Imagine that...

if it kept raining, until the entire Earth was flooded! I guess we'd have to build a boat real quick... like that guy Noah, in the old story... Actually it doesn't matter how much it's raining. Rain can never flood the entire Earth.

No, really! It's true. Not even if it's raining 24-7, for a year. Because every single drop of rain that falls on you here comes from an ocean or a lake somewhere else. The amount of water on the Earth always stays the same.

It's the same old water, that goes around and around... ... in an endless cycle. The rain that drips from your noses right now, continues down into the ground, beneath your feet. Some of that water is sucked up by the roots of plants. The plants use it to transport nutrients from their roots to their leaves and flowers.

Eventually, the plants let the water out again as water vapour, into the air. Some of the water stays in the ground for a while, as groundwater. We can pump that up by drilling or digging wells. The water that doesn't stay as groundwater runs through the ground and ends up in a lake or a river. We call this surface water, and we use it for all kinds of stuff.

Quite often we pollute the water we use. Eventually the water finds its way towards the nearest ocean. On its way, the water gathers salt and other substances that are buried in the ground. When the sun is shining on the ocean and heats it, some of the water evaporates. Water that evaporates transforms from liquid to gas, and becomes invisible water vapour.

When the water evaporates, the salt is left in the ocean, together with all the other stuff that it brought along. Some water also evaporates from lakes and moisture in the ground. Plants and animals release water and water vapour too. Warm water vapour is light, so it rises up into the atmosphere. Higher and higher, colder and colder.

Eventually it's so cold that the water can't stay in gas form. The vapour condenses and forms tiny water droplets. Many such water droplets together form a... Cloud. The water droplets fly around inside the cloud, and each time they bump into each other, they get a little bit bigger.

If this goes on long enough, the droplets get so heavy that they fall down, as rain. But most of the time they don't get that big. Because up in the clouds it's really cold. So the water forms crystals of ice instead -- snowflakes. Snowflakes fall, and if it's warm down on the ground, they melt on the way and turn into rain.

But if it's cold the snow lands on the ground, and stays until spring. Up on tall mountains - and at places where it's always cold - the snow doesn't melt during summer. Instead it is pressed together and forms - glaciers. The biggest glaciers are found on Greenland and at the South Pole. There, the water can stay for thousands of years before it melts and moves on again.

But sooner or later, the water comes back into the cycle. Same water, going round and around. So this drop of water, - it might have been... in... - MY MOUTH! - Or your GRANDAD's mouth! - Ewwww! That's correct.

Or your grandfather's father's mouth. Or your grandad's grandad's grandad's dad... or... Yes - thank you - the water on the Earth is old. It has travelled round the world and experienced more than any human has.

It's the sun that drives the water cycle, around and around. Evaporating from the ocean, forming clouds, raining down, being drunk, being peed out, running back to the ocean, then evaporating again.