* Water expands when it freezes: The molecules in ice are arranged in a crystal lattice that takes up more space than the molecules in liquid water. This is why ice floats.

* Density and temperature: Density is mass per unit volume. As water cools, it becomes denser until it reaches 4 degrees Celsius.

* Ice is less dense: Because ice is less dense than water, it floats.

Therefore, water at 4 degrees Celsius is denser than ice at 0 degrees Celsius.