Why do cooler substances have a tendency to sink?

Convection

Convection happens in fluids (liquids & gases). The heat energy moves through the material as the particles in the material move through the material.

Hot liquids and gases rise because when they are heated they expand and become less dense. The less dense warm liquid or gas then floats up through the more dense cold liquids and gases.

Cold liquids and gases sink because when cooled they contract and become more dense. The more dense cold liquids and gases sink down through the less dense warm liquids and gases.

These changes in density cause the convection currents in the liquid or gas.

That display data that change over time