Why do lakes freeze from the top?
The short answer
Water is densest at about 4°C. Colder water becomes less dense, stays near the surface, and can freeze there.
How it works
As surface water cools to about 4°C, it becomes denser and sinks. Below that temperature it becomes less dense, stays near the top, and freezes. Floating ice also insulates the water below.
A familiar example
This property helps keep the entire lake from freezing solid when ice covers the surface.