Question

In winter, water start freezing from the surface. Why?

Answer 1

Simply becoz of the low temp.

Answer 2

The density of the water is highest at 4oC. So the at winter the water having temperature 4oC goes to the bottom of the tank and the water having less temperature will be at the surface. Hence the freezing starts at the surface. Also since the ice is lighter than water ice also stays at the surface....