Drop an ice cube into a glass of water and watch. It bobs at the top! When water freezes into ice, it spreads out and takes up a bit more room. So a chunk of ice is lighter than the same-size chunk of water, and the lighter one floats.

Why does water spread out when it freezes?

Water is made of tiny bits called molecules. In runny water, they tumble about, packed close together. When water freezes, they lock into a neat pattern, like the six-sided one in a snowflake, with little gaps in between. Those gaps make ice take up more room.

Does everything float when it freezes?

When most things freeze, they pack closer together. So a solid chunk of them sinks in their own liquid. Water does the opposite, and its ice floats. That's why a pond freezes on top, where you can see the ice.

Why does ice float?

Roo answers for ages 4–11, then goes 2 steps deeper each time a child asks why.

Roo says

Drop an ice cube into a glass of water and watch. It bobs at the top! When water freezes into ice, it spreads out and takes up a bit more room. So a chunk of ice is lighter than the same-size chunk of water, and the lighter one floats.

For ages 4–7

Checked against Water density (USGS)
Why does water spread out when it freezes?

Water is made of tiny bits called molecules. In runny water, they tumble about, packed close together. When water freezes, they lock into a neat pattern, like the six-sided one in a snowflake, with little gaps in between. Those gaps make ice take up more room.

For ages 6–9

Checked against Water density (USGS)
Does everything float when it freezes?

When most things freeze, they pack closer together. So a solid chunk of them sinks in their own liquid. Water does the opposite, and its ice floats. That's why a pond freezes on top, where you can see the ice.

For ages 8–11

Checked against Water density (USGS)