Why do lakes freeze from top down?

Answer: Ice is less dense than water

Ice is less dense than water ✓ — Correct! Lakes freeze top-down because ice floats. Water is most dense at 4°C, and as it cools further it becomes less dense. At 0°C it freezes into ice, which is even less dense, so it floats on the liquid water below. That floating ice insulates the deeper water, keeping it liquid so fish can survive. If ice sank, lakes would freeze solid and aquatic life would be impossible.

Deep water is naturally warmer — Wrong. Deep water can stay around 4°C because that's the temperature at which water is most dense, but ice forms on top because it's less dense and floats, not because depth is inherently warmer.

Wind cools the surface first — Wrong. Wind can speed up surface cooling, but the fundamental reason ice forms on top is that ice is less dense than water, so it floats and insulates the water below.