Molecules in an ice cube have lower kinetic energy than molecules in a radiator.
Molecules in an ice cube move more slowly than molecules in a radiator. They collide with less force than molecules in a radiator.
When in direct contact, the fast-moving molecules in the radiator collide with the slow-moving molecules in the ice cube.
Kinetic energy is transferred from the molecules in the radiator to the molecules in the ice cube, causing a slowdown of the radiator molecules and an acceleration of the ice molecules.
