Freeze most liquids and the solid sinks. Molten wax, molten metal, solid alcohol dropped back into liquid alcohol — cooling packs molecules tighter, tighter packing means more mass in the same space, and denser things sink under lighter ones. That's the rule almost everywhere in chemistry. Water breaks it. Ice floats on liquid water, which means the solid form of the most common liquid on Earth is less dense than the liquid it came from. That's the anomaly, and it isn't a small one.
A molecule with a job to do at both ends
A water molecule is bent, not straight, and the bend matters: the oxygen end carries a slight negative charge and the two hydrogen ends carry a slight positive one. That lopsided charge lets one water molecule's hydrogen reach for the next molecule's oxygen — a hydrogen bond, weaker than the bond holding the molecule itself together, but strong enough to matter in bulk. In liquid water, these bonds form and break constantly, molecules sliding past each other while still tugging on their neighbours.
Cool the liquid down and the molecules stop sliding long enough for the hydrogen bonds to lock into a fixed pattern — and that pattern is the anomaly's whole cause. Each water molecule can hydrogen-bond to exactly four neighbours, and the geometry that satisfies all four at once is an open, tetrahedral lattice with empty space at its centre. Ice isn't water molecules packed as tightly as they'll go. It's water molecules held apart, in formation, because the bonds insist on a specific angle and won't bend to save space.
What floating ice actually buys a planet
This isn't a trivia footnote; it's load-bearing for how liquid water survives winter at all. If ice sank, a freezing lake would fill from the bottom up — cold, dense ice settling on the floor while liquid water above kept radiating heat into the air, until, in a hard enough winter, the whole thing froze solid from the bottom to the top. Instead, ice forms a floating lid. That lid insulates the water underneath from the air above, so a lake can carry a metre of ice on its surface and still be liquid, and survivable for whatever's living in it, a few feet down.
What we're still unsure about
It's tempting to file this as "water is weird" and leave it there, but the honest version is narrower: water is weird specifically about the relationship between its solid and liquid densities, driven by one particular geometric fact about hydrogen bonds at four-per-molecule. Water has other odd properties too — an unusually high boiling point for such a small molecule, a density that peaks not at freezing but a few degrees above it — and it's worth being precise about which anomaly is doing the explaining in any given case, rather than waving at "hydrogen bonding" as a catch-all for everything unusual about water.
This sits inside States of Matter & Intermolecular Forces, one of eight topics in General Chemistry, one of six domains in Chemistry, one of seventeen subjects the app can quiz you on.