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- [Instructor] Did you know
that the water you drink

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is actually the same
water that dinosaurs drank

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over 65 million years ago?

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It might be hard to believe

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but your water is actually
really, really old.

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In fact, water on Earth is
much older than the dinosaurs.

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Scientists estimate
that the water on Earth

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is at least 4.6 billion years old.

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And the amount of water on Earth today,

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in lakes, rivers, oceans, glaciers,

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even under the ground
and up in the clouds,

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it's about the same as it was

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millions and millions of years ago.

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That's because water is recycled.

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It just gets used again and again.

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And that brings us to the water cycle,

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which is how water continuously moves

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from the ground to the
atmosphere and back again.

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And as water moves through
the cycle, it changes form.

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In fact, water is the
only substance on Earth

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that naturally exists in three states,

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solid, liquid, and gas.

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Have you seen water in all
of its different states?

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Maybe on a hot day, you'll add some ice,

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which is water in its solid state,

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to a glass of liquid water.

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Or maybe when you take out some food

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that you've heated in the microwave,

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you'll see steam coming off of the food,

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which is water in its
gas state as water vapor.

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When you think of water,

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you might think of the wide open ocean.

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Over 95% of all the water
on Earth is in the ocean,

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so this is a great place to
start with the water cycle.

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Here, energy from the sun warms up water

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on the surface of the ocean

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enough to turn it into water vapor.

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This is called evaporation.

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This water vapor is less dense,

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meaning it's lighter, than liquid water,

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so it rises up and up into the atmosphere.

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However, as the water vapor rises,

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the temperature in the atmosphere cools.

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In turn, the water vapor condenses

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into tiny liquid water droplets,

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or, as we see them, clouds.

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This is called condensation.

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Air currents then move these
clouds all around the Earth.

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As a cloud collects more and
more liquid water droplets,

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the water may be released from the cloud,

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pulled down by gravity,

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and then return to the ocean or land

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as precipitation, like rain.

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If it's really cold, though,

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the water drops may
crystallize and become snow.

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The snow will fall to the ground

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and eventually melt back into a liquid

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and run off into a lake or
river, pulled down by gravity,

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which flows back into the ocean

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where the whole process starts over again.

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But that's just one path water can take

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through the water cycle.

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It's like a choose your own adventure.

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Instead of snow melting and
running off into a river,

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the snow could become part
of an icy cold glacier

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and stay there for a long, long time,

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for thousands of years.

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Or rain can seep into the
ground and become groundwater,

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where it's then absorbed by plant roots.

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In turn, through transpiration,

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the water absorbed by the plants

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can transition to water vapor

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and leave directly through the leaves

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via tiny holes called stomata

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and return to the atmosphere.

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Or instead of being
absorbed by plant roots,

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the groundwater can work its
way to an underground aquifer

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or a lake, river, or even the ocean.

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There are many different paths for water

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and the water cycle can
be very complicated.

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But it really comes down
to something very simple.

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The amount of water on Earth

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stays pretty constant over time

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and moves from place to place,

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sometimes transitioning between phases,

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depending on things
like weather, geography,

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solar energy, and gravity.

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Now, we know that water is
essential to life on Earth

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and fresh water is an
especially limited resource

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for a growing world population.

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Changes in the water
cycle can impact everyone

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through the economy, energy production,

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health, recreation
transportation, agriculture,

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and of course drinking water.

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And that's why understanding
the water cycle

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is so important.

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That, and it's pretty cool to know

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that you drink the same
water as dinosaurs did.

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Until next time.