Here's why:

* Conduction: Heat transfer through conduction occurs when molecules directly collide with each other, transferring energy from hotter molecules to colder ones. This requires a medium, like a solid, liquid, or gas, where molecules are in close proximity.

* Convection: Involves the movement of fluids (liquids or gases). Heat is transferred by the movement of these fluids, which carry heat energy with them. While convection does involve molecules, the transfer of heat primarily occurs through the movement of the fluid itself, not directly between molecules.

* Radiation: Heat transfer through radiation doesn't require molecules at all. It involves the transfer of energy through electromagnetic waves, which can travel through a vacuum.

In summary: Conduction is the only type of heat transfer that directly depends on the collisions of molecules.