Ice floats because water is a weirdo

Almost any substance grows denser when it solidifies, so its solid form sinks in the liquid. Water is the rebel: ice is less dense than liquid water, which is why it floats. An ice cube in your glass is a physical oddity in miniature.

The reason lies in how its molecules bond. On freezing, water arranges them into a wide-open hexagonal lattice, with more gaps than when it's liquid. More space means less density. In fact water reaches its maximum density at around 4 degrees, still liquid, not at zero.

And this isn't a pretty quirk: it holds up life. Lakes freeze from the top, and that layer of ice insulates the water beneath, which stays liquid. Fish and plants get through the winter down there. If ice sank, lakes would freeze solid from the bottom. It floats, and in doing so it saves us.