ELI5: Why does ice float?

Imagine you have a box of toys.

Normally, when you pack toys tightly, they take up less space, right? That's how most things work. When they get colder, they shrink and become more packed together.

But water is special!

Think of water molecules like tiny bouncy balls.

• When water is liquid (like in your glass), the bouncy balls are moving around and can squeeze closer together. They're packed pretty well.

• When water freezes into ice, the bouncy balls turn into tiny LEGO bricks. These LEGO bricks can only fit together in a specific way.

Since the ice takes up more space, even though it's still the same amount of water, it becomes less dense. Density means how much stuff is packed into a space.

Imagine you have a pillow and a rock. The rock is much heavier (more dense) than the pillow, even if they are the same size.

Because ice is less dense than liquid water, it floats! It's like the pillow on top of the rock. The less dense ice sits on top of the more dense water.

That's why ice cubes float in your juice, and why icebergs float in the ocean! If ice sank, lakes and oceans would freeze from the bottom up, which wouldn't be good for fish or anyone else!