Now, the main idea, the main conclusion that we get from quantum mechanics is the following tiny subatomic particles called electrons and protons and neutrons and other particles. Now, subatomic simply means smaller than an atom gain or lose energy in discrete amount called photons. Now, we're going to talk more about photons and what they are when we'll talk about the photoelectric effect. Now, this statement is analogous to the following everyday situation. Suppose we have a vending machine. And this vending machine only accepts coins that are $0.25.