Here's why:

* Water expands when it freezes: This is why ice floats. As water cools, its molecules slow down and become more tightly packed, increasing its density.

* Maximum Density: Water reaches its maximum density at 4°C (39.2°F). Below this temperature, the density starts to decrease again due to the formation of ice-like structures.

It's important to note that this is the densest form of water under normal atmospheric pressure. Under extreme pressure, the density of water can be increased further.