Re: Why does heat transfer occur?

The  temperature of an object is a measure of how fast the molecules or atoms are moving 
around.  The hotter the object, the more the molecules are moving.

Heat transfer is really just a transfer of that motion.  In conduction or convection, the hotter 
object's molecules bump into the molecules of the cooler object at the interface. Each time, 
momentum is transferred.  So the hotter molecule slows down a bit and the cooler object 
molecules speed up.  This transfer occurs even among the cooler and warmer molecules 
within the object.  Eventually, they all are moving at the same average speed and we say that 
the temperature has reached an equilibrium.  This temperature is between the original hot and 
cold body temperatures but in the ratio of their initial mass as well.

Heat transfer by radiation is a bit more difficult to convey because the energy transfer occurs 
by radiation.  In that case, the hotter molecules emit radiation or infra red light which is 
absorbed by the slower, cooler molecules.

Hope that helps.