Thermodynamics is a branch of science that deals with heat, work, and energy. It tells us how heat can be transferred between objects, how energy can be converted from one form to another, and how work can be done with energy. The three main principles of thermodynamics are the first law, which states that energy cannot be created or destroyed, only transferred or transformed; the second law, which states that heat can only flow from hot to cold; and the third law, which states that it is impossible to reach absolute zero, the coldest temperature possible.