system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Why does water expand in volume when freezed? Shouldn't it be more compact now since it is a solid?
gpt
In most cases, when something freezes,the molecules pack more tightly than they are on average when flowing around each other as a liquid.

In the case of water, however, the molecules form layer upon layer of a hexagonal crystal structure that encloses about 10% more volume that the molecules take up in liquid form. This means that ice is *less* dense than liquid water.