In 1905, Albert Einstein published his theory of Special Relativity. In this paper, Einstein took his readers through the thought process which led him to the most famous equation, E = mc².
Introduction to mass and energy
Mass is a measurement of how much quantity of matter inside of an object. Mass is also a measure of inertia - the more mass an object has, the harder it is to move and once it is moving, the harder it is to stop it. On the hand, energy is the ability to do work. Mass and energy can be neither created nor destroyed. While they can be converted into different forms (mass can turn from solid to liquid to gas, and can be cut up or turned to dust, while kinetic energy can be transferred to potential energy or sound energy or heat energy), there will always remain the same amount in our universe.