Heat transfer is a subject that deals with the generation, use, conversion, and exchange of thermal energy between a system and its surroundings. At the molecular level, we can visualize thermal energy by the motion of particles and quantify it by the overall kinetic energy of all the particles. Heat transfer can then be explained as an exchange of the kinetic energy among particles via collisions or interactions. In a gas, the particles move faster, on average, under higher temperatures and transfer their kinetic energy to particles at lower temperatures through collisions. In a solid, the particles vibrate faster under higher temperatures and transfer their kinetic energy to particles at lower temperatures through molecular interactions.