Final answer:
A rock stays afloat on a frozen lake because the ice layer is less dense than the water beneath, acting as an insulating layer that supports the rock and insulates the water below.
Step-by-step explanation:
A rock does not sink in a lake that has been frozen over because ice is less dense than liquid water, which causes it to float on the surface. When a lake freezes, a protective insulating layer of ice forms on the top. This insulating layer supports objects, such as rocks laying on the surface and prevents them from sinking. It also allows aquatic life to survive underneath by maintaining a liquid environment insulated from the freezing temperatures above.