Density changes based on temperature, meaning cold water molecules pack tightly together and sink, while hot water molecules spread out and rise to the surface.

A closer look

Density changes based on temperature, meaning cold water molecules pack tightly together and sink, while hot water molecules spread out and rise to the surface. However, water reaches its maximum density at exactly four degrees Celsius, after which freezing turns it into ice, causing it to expand and miraculously float. If water didn't have this weird, rule-breaking property, lakes would freeze from the bottom up, violently killing all marine life every single winter. Physics essentially coded a last-minute patch into the universe's software just to make sure fish don't get trapped in solid ice blocks. It is comforting to know that even the fundamental laws of thermodynamics have to make awkward exceptions to avoid completely destroying the entire ecosystem