Here's why:

* Heat is a form of energy.

* Temperature is a measure of the average kinetic energy of the particles in a substance.

* When two objects with different temperatures are in contact, the particles with higher kinetic energy (the warmer object) collide with the particles with lower kinetic energy (the cooler object).

* This collision transfers energy from the warmer object to the cooler object, resulting in the warmer object losing heat and the cooler object gaining heat.

This continues until both objects reach a thermal equilibrium, meaning they have the same temperature.