It evaporates off the sea, drifts up as cloud, falls as rain, and runs back to the ocean — the same water, going round and round, powered by the Sun, for billions of years.
Start hereThe mouthful of water you sipped this morning has been on an almost unimaginable journey. It has been ocean, and cloud, and rain, and river, over and over — recycled so many times that some of it has very likely been cycling around the planet since long before there were dinosaurs to drink it.
That endless reusing is the water cycle: the way water moves in a giant loop between the sea, the sky, and the land. Nothing is added; nothing is thrown away. The same water simply changes its outfit — liquid, then invisible gas, then tiny cloud droplets, then liquid again — and travels.
And the whole loop runs on one engine: the Sun. Sunlight lifts water off the ocean as vapour, and from there gravity and weather do the rest, carrying it up, across, and back down. No Sun, no cycle. With it, the planet's water has been turning like a slow, silent wheel for an almost incomprehensible stretch of time.
By the end of this page you'll be able to do something genuinely useful: look at a cloud, a puddle, a river, or steam rising off a hot drink, and know exactly where that water came from and where it's headed next. So let's start by turning on the engine and watching the wheel turn.
Picture the ocean on a hot, bright day. Sunlight pours down and warms the very top layer of the water. Warming gives the water molecules more energy — they start jostling faster — and the fastest ones near the surface break free and float off into the air as an invisible gas called water vapour. That escape is called evaporation, and it's the first step of the whole cycle.
Here's the part worth really holding onto: the Sun is doing lifting work. It takes water that was lying flat and heavy in the sea and, molecule by molecule, sends it floating high into the sky. That's a bit like an invisible elevator running on sunlight, carrying water upward all day, everywhere the Sun shines. Everything that happens next — the clouds, the rain, the rushing rivers — is that lifted water making its way back down.
So the more sunshine you pour in, the faster water gets lifted, and the faster the whole loop spins. You don't have to take my word for it. In the demo below, the Sun is a slider. Turn it up and watch the entire cycle speed up — more vapour rising, clouds filling faster, rain coming sooner, rivers running harder. Turn it down and everything slows to a sleepy crawl.
This is one little world: sea on the left, mountains and a river on the right, sky in between. Watch the water move in a loop — up off the sea as vapour, into the cloud, down as rain, and home down the river. The label that lights up tells you which stage is happening right now. Now grab the Sun slider and crank it: the whole wheel spins faster.
A warm Sun keeps the water turning at a steady, gentle pace.
Did you spot the loop close itself? A drop that evaporates off the sea on the left eventually comes home down the river on the right — and then it's back in the sea, ready to be lifted all over again. Nothing leaves this little world. The Sun just keeps pushing the same water around the track, and the four labels — evaporation, condensation, precipitation, collection — are the four corners every drop passes through on its lap.
Most of the water rising into the sky comes straight off the oceans, because that's where most of Earth's water is. The Sun warms the surface, the speediest molecules slip free as vapour, and they float upward — invisibly. You can't see water vapour; it's a clear gas, just like the air around you. The steam you spot above a kettle or a hot shower isn't the vapour itself — it's vapour that has already cooled a little and turned back into tiny droplets you can see.
It's not only the sea that breathes out, either. Puddles shrink and vanish on a sunny day, wet clothes dry on a line, a damp playground turns dry by lunchtime — all evaporation, all the Sun lifting water into the air. Even plants join in: they quietly release water vapour from their leaves, a gentle version of the same thing called transpiration. A big forest can puff an astonishing amount of water into the sky this way. Add it all up — oceans, lakes, puddles, soil, and leaves — and the planet is exhaling water into the air every single second the Sun is up.
In the demo, evaporation is the pale wisps drifting up off the sea. Push the Sun slider higher and you'll see more of them, rising faster. That's the truth of it: heat is the ticket up. The warmer the water and air, the more water evaporates.
Up the vapour goes — but the higher you climb in the sky, the colder it gets. And cold is the opposite of what set the water free in the first place. Chilled down, the vapour loses energy, slows back down, and the molecules clump together again into tiny specks of liquid water. That switch from invisible gas back to liquid is condensation, and it's how clouds are born.
This clears up one of the most common mix-ups about the whole cycle, so let's say it plainly: a cloud is not water vapour you can see. Water vapour is invisible. A cloud is made of countless tiny liquid droplets (and, up high, tiny ice crystals) — so small and light that they float, like dust caught in a sunbeam. When enough of those droplets gather in one patch of sky, that patch turns white and fluffy, and we call it a cloud. So a cloud is really a crowd: billions of microscopic drops, hanging out together where the air got cold enough to make them.
Watch this happen in the demo. The vapour wisps climb, reach the cool air up high, and gather into the cloud — which grows fuller and a little greyer as more and more droplets pile in. A cloud, in other words, is a bucket slowly filling. And a bucket that fills up has to eventually spill.
A kitchen version you've seen a hundred times. Take a cold drink out on a warm day and the glass "sweats." That water didn't leak through the glass — it's vapour from the warm air touching the cold surface, cooling down, and condensing into liquid droplets right before your eyes. A cloud is the same trick, just happening high in cold air instead of on your glass.
Cloud droplets are unbelievably tiny, and at first they're light enough to float. But inside a cloud they keep bumping into each other and merging — two droplets becoming one bigger drop, again and again. Sooner or later a drop gets too heavy for the air to hold it up, and gravity wins. Down it falls. Water falling from the sky — in any form — is called precipitation.
Usually that's rain: liquid drops. But if the air is cold enough, the water freezes on the way and arrives as snow, or as hail or sleet. It's all the same water coming back to Earth — just dressed for the weather. High on cold mountains, the cloud often hands its water back as snow, which can pile up and wait for months before it melts and joins the journey again.
In the demo, precipitation is the streak of rain falling from the full cloud onto the mountainside. Notice the timing: with a strong Sun, the cloud fills quickly and rain comes fast; with a weak Sun, you wait and wait for the cloud to be ready. Either way, the water that the Sun lifted off the sea is now landing back on the land — and the only direction left to go is down.
Once rain or melted snow lands on the ground, it begins the trip back to the sea — and water has a simple rule for travel: it always flows downhill. Drops trickle together into little streams, streams join into rivers, and rivers carry the water down off the mountains and across the land all the way to the ocean. This flowing-back-together is called collection, and the water sliding over the surface on its way is called runoff.
Not all of it travels on the surface, though. Some water soaks down into the soil and rock and becomes groundwater, hidden underground. It moves much more slowly down there — sometimes taking years — but it, too, is creeping steadily toward the sea, and some of it bubbles back up as springs that feed the rivers. Whichever path it takes, the destination is the same: back to the ocean, where the Sun is waiting to lift it skyward and start the whole lap over.
In the demo, watch the rain land on the mountain and then run down the river back into the sea. The moment it arrives, it's "collected" again — and a drop in the sea is exactly where we started. The loop is closed. That's the single most important word for the water cycle: it's a loop, with no beginning and no end, just water going round.
A drop doesn't zip around the loop non-stop. It pauses — sometimes for a heartbeat, sometimes for ages — in the places water gets stored. Knowing the main storerooms makes the whole cycle click into place:
Compared with those three giants, the water in lakes, rivers, and the air above us is a surprisingly thin slice — but it's the slice that moves the fastest and that we see the most. A drop might spend a single day as a cloud, a week in a river, but thousands of years frozen in a glacier or resting deep underground. So when you sip a glass of water, you're catching a traveller in the middle of a journey that might have included a very, very long nap.
Here's a lovely twist: the water itself doesn't care whether it's ocean, ice, cloud, or river. It's the same H2O the whole time. "Ocean" and "glacier" and "cloud" aren't different waters — they're just the same water, parked in different places, waiting for its turn to move again.
Meet Dewdrop. Right now she's floating in the ocean. Press Next stage (or tap any step) to ride along as she takes one full lap of the cycle — sea, sky, cloud, rain, and back. When you reach the end, the next tap drops her straight back in the sea, because the journey truly never stops.
Notice there's no finish line. Dewdrop's "end" is just the sea again — the exact spot she started — so the next thing that happens is she gets lifted right back into the sky. That's what people mean when they say the water cycle is endless: every drop's journey is a circle, and a circle has no last step.
If water is being used all the time — drunk, rained out, frozen, flushed, evaporated — why doesn't the planet run out? Because of the one word we keep circling back to: it's a loop. The water cycle doesn't spend water; it moves it. Every drop that leaves the sea comes back. Every cloud that empties gets refilled. The total amount of water on Earth stays just about the same, year after year — it simply keeps changing form and changing places.
This is also why the framing at the very top is true: the water you drank today is genuinely ancient. Since Earth doesn't make new water and the old water never leaves the loop, the same water has been cycling around the planet for an almost unimaginable stretch of time. The drop in your glass has very likely been a cloud over an ocean that no longer exists, a river that dried up long ago, and a snowflake on a mountain — countless times. You're not drinking new water. You're borrowing extremely old water for a moment, and soon it'll be back on the road.
That leads to the last big misconception worth busting. A lot of people quietly assume that rain is "fresh" or "new" water the sky somehow makes. It isn't. Rain is recycled — it's sea water and lake water and puddle water and plant-breath that the Sun lifted up, that condensed into a cloud, and that fell back down. The cloud didn't create it; the cloud just held it for a while. Every raindrop is a returning traveller, not a newborn.
So is the cycle perfectly sealed? Almost. Tiny amounts of water do get locked into rocks or split apart high in the atmosphere, and a trickle of brand-new water arrives over the ages, but these are tiny next to the vast ocean of water already circling. For everyday purposes, treat Earth's water as a fixed amount on an endless loop — that's the right mental picture.
No pressure — pick an answer and we'll explain it either way. This is how it sticks.
Sunlight evaporates water off the sea and land, sending it up as invisible vapour.
The vapour cools and condenses into clouds, then falls back as rain or snow.
It runs downhill through rivers and rock back to the sea — and the loop begins again.