Question

In cold countries lakes and ponds freeze only at the surface while well below the surface water remains in liquid state at 4°C.... WHY?????

Answer 1

As the temperature of water drops, it contacts,its density increases, and hence, it sinks to the bottom of tank.This continues till 4°C

Answer 2

it is that,warm water gets more dense when it is cooled and during summers the top surface water gets directly exposure to sun and this makes it more dense when cooled but in deep the water is not so warm and so it is less dense when cooled. also the warmer water sinks due to high density than cold water as this water is less dense they expand themselves to form ice.
this continuous till 4*c
       ;hope the ans would help u...