# What is so special about water at 4^@ C?

Because at $1$ $a t m$, this is the $\text{densest}$ state that water exhibits, i.e. with respect to solid, or liquid (or of course gas).
And given this density, $\text{ice}$ will float in water. That the liquid phase is denser than the solid phase is an highly unusual phenomenon, that is shared by few other materials.