What is the source of heat?
Heat itself is the motion of atoms, molecules and other particles. The faster it’s moving the hotter it is. Think of a metal spoon. The spoon is made up of slow moving molecules. If you put that spoon into a very hot bowl of soup, you soon notice the handle of the spoon is warm, if the soup is hot enough the spoon may become too hot to hold. The molecules in the soup are traveling at such high speeds that when the come into contract with the spoon, they start making the molecules in the spoon move faster. The molecules in the spoon all hit each other so that even the handle will warm.
Now lets put a piece of ice in the soup. The molecules are bouncing off the ice making the ice warm up, but the soup molecules are being slowed down when they come into contact with the ice. The same thing happens with the spoon, but since some substances transfer heat quicker than others (the spoon is faster than the ice) the ice is better at cooling the soup, while the spoon is better at getting hotter.
What this all boils down to (pun intended) is that heat is nothing more than energy. Heat is only one form of energy and when fast (or hot) atoms or molecules hit another molecule some of that energy is transferred. Another good example is a car accident. If one car hits another from behind the first car will move forward. That’s the transfer of kinetic energy.
Unless you’re referring to this Heat, in which case the source is Michael Mann.