This chapter deals with the important topic of Physics, which called as “Heat”. Heat is energy, it's transferred from one body to another as the result of difference in temperature or even created at the expense of the loss of other forms of energy. Heat energy is the result of the movement of tiny particles called atoms, molecules or ions in solids, liquids and gases. Heat energy can be transferred from one object to another. The transferor flow due to the difference in temperature between the two objects is called heat if two bodies at different temperatures are brought together, energy is transferred—i.e., heat flows—from the hotter body to the colder. The most prominent example of heat energy in our solar system is the sun itself.