Why does the bottom of a lake not freeze?
Water has a remarkable characteristic.
It has its highest density at 4 °C.
This is when it is heaviest.
Not when it freezes, which is at 0 °C.
When the surface of a lake cools down to 4 °C
it sinks to the bottom.
If the air temperature cools the lake further,
the colder surface water becomes lighter.
It lies as a layer on top of the four-degree water.
This is why lakes freeze from the surface and then downwards.