A child at a kitchen table looks closely at a tall glass of blue drink with a single teal ice cube bobbing right at the top of the surface.

Why does ice float?

A rock sinks. A frozen rock sinks even harder. But freeze water and the cube bobs right on top of your drink. Same stuff — so what changed?

Floating is really a weigh-off. Put two same-size cups on a scale — the lighter one always rides up on top. So the sneaky question is: which cup holds less?

A simple balance scale holding two identical cups: the cup crammed full of blue dots hangs low while the cup with only a few dots rides high up in the air.

Water bits are tiny V-shapes with sticky corners. Warm bits jiggle too hard for the corners to hold on, so they tumble in a loose huddle. Tap to calm them: the sticky corners finally grab, and the bits lock hands into a stiff, roomy ring.

tap the water bits

Same cup, two ways to pack. Jiggly bits don't hold a pattern — they squeeze into a tight huddle, so a LOT crowd in. Frozen corners grip corner-to-corner, and that grip only fits one way: stiff open rings with holes — far FEWER fit.

Two identical cups side by side: the left cup is packed completely full of blue water bits, while the right cup holds far fewer teal bits arranged in open six-sided rings with empty holes.

Your turn — be the freezer. Drag the lever to freeze and melt the left cup.

left cup: frozen ice
MELTEDFROZEN

Cold calms the jiggle — the corners grab and rings lock. Warmth breaks the grip; the huddle packs tight again.

Now the real test.

Two identical cups sit side by side on a table, not on any scale: one is a frosty block of teal ice, the other holds liquid blue water, with a big question mark floating above them.

We freeze one cup solid and keep one cup liquid — same-size cups, filled to the same line. When a balance is released, which side will rise? 🤔

Solids usually feel heavier than their melty version.

Put two same-size cups on a level balance — one liquid water, one solid ice. When you let go, what happens?