Why does ice float?

Pop an ice cube into your glass and watch it bob at the top. But a stone or a coin would sink straight down. Ice is just frozen water — so why does it float instead of sinking?

Light things float

First, a little rule: whether something floats depends on how heavy it is for its size. Things that are light for their size float, and things that are heavy for their size sink. A big beach ball is light for its size, so it bobs on the water.

Freezing makes water spread out

Here is the surprise hidden inside an ice cube. When water freezes, the tiny pieces it is made of line up and push apart, leaving little gaps. So the water spreads out and takes up more room than before.

It is still the very same water — but now it fills a bigger space. That makes ice lighter for its size than the water around it.

So ice floats!

Because ice is lighter for its size, it cannot sink — it rises up and floats on top. That is why ice cubes bob in your drink and why sheets of ice form on the top of a cold pond, not the bottom.

Wonder fact: Floating ice is a lifesaver! In winter the ice stays on top of a pond, and the fish swim happily in the unfrozen water below.