When a liquid warms up it swells and becomes lighter. We say the warmed liquid is less dense. When it gets cold it shrinks and becomes heavier.
As a result, hot liquids rise and cold liquids sink.
When we heat a saucepan on the bottom, the water in it rises to the top and cold water takes its place and is heated in turn. In this way, the water churns over and all gets to the same temperature.
