The type of heat transfer that involves molecules bumping into each other is conduction.

Here's why:

* Conduction is the transfer of heat through direct contact between molecules. When a substance is heated, its molecules vibrate faster and collide with their neighbors, transferring energy. This process continues throughout the material, causing the entire substance to heat up.

* Convection involves the transfer of heat through the movement of fluids (liquids or gases). This movement is driven by differences in temperature and density, creating currents that carry heat.

* Radiation involves the transfer of heat through electromagnetic waves, which can travel through space or even a vacuum.

Let me know if you'd like more explanation on any of these types of heat transfer!