The water cycle | Weather and climate | Middle school Earth and space science | Khan Academy

Did you know that the water you drink is actually the same water that dinosaurs drank over 65 million years ago? It might be hard to believe, but your water is actually really, really old. In fact, water on Earth is much older than the dinosaurs. Scientists estimate that the water on Earth is at least 4.6 billion years old.

The amount of water on Earth today—in lakes, rivers, oceans, glaciers, even under the ground and up in the clouds—it's about the same as it was millions and millions of years ago. That's because water is recycled; it just gets used again and again. And that brings us to the water cycle, which is how water continuously moves from the ground to the atmosphere and back again.

As water moves through the cycle, it changes form. In fact, water is the only substance on Earth that naturally exists in three states: solid, liquid, and gas. Have you seen water in all of its different states? Maybe on a hot day, you'll add some ice—which is water in its solid state—to a glass of liquid water. Or maybe when you take out some food that you've heated in the microwave, you'll see steam coming off of the food, which is water in its gas state as water vapor.

When you think of water, you might think of the wide open ocean. Over 95% of all the water on Earth is in the ocean, so this is a great place to start with the water cycle. Here, energy from the sun warms up water on the surface of the ocean enough to turn it into water vapor. This is called evaporation. This water vapor is less dense, meaning it's lighter than liquid water, so it rises up and up into the atmosphere.

However, as the water vapor rises, the temperature in the atmosphere cools. In turn, the water vapor condenses into tiny liquid water droplets, or as we see them, clouds. This is called condensation. Air currents then move these clouds all around the Earth. As a cloud collects more and more liquid water droplets, the water may be released from the cloud, pulled down by gravity, and then returned to the ocean or land as precipitation, like rain.

If it's really cold, though, the water drops may crystallize and become snow. The snow will fall to the ground and eventually melt back into a liquid and run off into a lake or river, pulled down by gravity, which flows back into the ocean where the whole process starts over again.

But that's just one path water can take through the water cycle. It's like a choose your own adventure. Instead of snow melting and running off into a river, the snow could become part of an icy cold glacier and stay there for a long, long time, for thousands of years. Or rain can seep into the ground and become groundwater, where it's then absorbed by plant roots.

In turn, through transpiration, the water absorbed by the plants can transition to water vapor and leave directly through the leaves via tiny holes called stomata and return to the atmosphere. Or instead of being absorbed by plant roots, the groundwater can work its way to an underground aquifer or a lake, river, or even the ocean.

There are many different paths for water, and the water cycle can be very complicated. But it really comes down to something very simple: the amount of water on Earth stays pretty constant over time and moves from place to place, sometimes transitioning between phases depending on things like weather, geography, solar energy, and gravity.

Now, we know that water is essential to life on Earth, and freshwater is an especially limited resource for a growing world population. Changes in the water cycle can impact everyone through the economy, energy production, health, recreation, transportation, agriculture, and, of course, drinking water.

That's why understanding the water cycle is so important. That, and it's pretty cool to know that you drink the same water as dinosaurs did. Until next time!