What is anamoulus expansion?

The expansion of water takes place as it is cooled from 4⁰C to 0⁰C. This behavior is unusual because most substances contract when they are cooled. This shows that water has a greater density at 4⁰C than at 0⁰C. Hence, while most substances become more dense as they solidify from liquid state by freezing, water becomes less dense on solidifying.This unusual expansion of water is termed as anamolous expansion of water. This is the reason why ice floats on water.... as its density is less than that of water.

This is why ice floats on water and the water at the bottom of a pool in winter is warmer than at the surface.