Compare the heating in the mantle to a cooking pot of soup.

The heating of the mantle could be compared to cooking a pot of soup by taking this situation into consideration:

When a soup gets heated, it actually spreads inside the pot and then cools down as the heat gets removed. Similarly, the mantle has this convection currents that when it gets heated, this current causes the plates to move and cools down after. The plate tectonics could be similarly explained using this phenomenon.