An ice cube floats in a glass of water. Sanvi says ice floats because it is a lighter substance…

Why

Sanvi's comparison to wood is a reasonable one — wood really is a different, lighter material than water, so it makes sense to guess ice is too. But ice and liquid water aren't two different substances at all — ice IS water, just frozen. What actually happens when water freezes is that its particles lock into a spread-out, roomier pattern than they had as a liquid, so the same amount of water takes up slightly MORE space as ice. Taking up more space for the same amount of matter means ice is very slightly less packed — less dense — than liquid water, and that small difference is exactly enough to let it float. Test: freeze water right to the brim of a sealed bottle and it will bulge or even crack the bottle, because it expanded on freezing — proof it takes more room frozen than liquid, using the very same water throughout. No, the reason is different: ice isn't a separate lighter material, it's the same water arranged in a way that takes up a little more space.