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

- [Instructor] 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.

And 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.

And 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 return 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 fresh water 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.

And 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.