Water is densest at 4∘C; this is the reason lakes do not completely freeze during extreme winters. Say, the atmosphere is at -θ∘C(θ>0),now the temperature of the lake's water starts to fall, and the denser water from the top sinks, getting the bottom layers up, and cooling them until the temperature reaches 4 ∘C at the upper surface. Now, further reduction in temperature actually makes the water less dense! Hence the colder water stays on top, starts to freeze, until the top layer of ice reaches -θ∘C and the bottom layer at 0 ∘ C . And this layer of ice is the only area through which the water below loses heat (by conduction), slowly increasing the thickness of the ice (because ice is a bad conduction), and for a very long time the water at the bottom of the lake remains at 4 ∘ C! Now, Let the thickness of the layer of ice be y1, at an instant.How much time t, will it take to increase to thickness y2? ρ→ density of the ice K → thermal conductivity of ice. L → Latent heat of fusion θ→ temperature of surface (as mentioned earlier)