 So in this module we're going to talk about some of the most important properties of particles. The first one that we're going to talk about is something called spin. Spin's kind of a funny quantity. It's kind of like angular momentum except it's intrinsic to a particle. So that means you can't change it. Spin is given by a spin quantum number times h-bar, where h-bar is Planck's constant over 2 pi. Now what's important about spin is that the spin quantum number is quantized. So what that means is it can either be half integer or integer. It can only come in discrete quantities. There's discrete steps. This is as opposed to continuous quantities where you could have any value for us. So this spin quantum number is a quantum property of a particle. I've noted that spin can either be half integer or integer. We have special names for the particles that fall into each of these categories. So the half integer particles we call fermions, and the integer particles we call bosons. One of the common fermions that you can think of are electrons. One of the common bosons that you would be familiar with is a photon. We'll learn about lots of different types of fermions and bosons later on, but for now we'll leave it there. Fermions are half integer spin, bosons are integer spin. The reason spin is important is that there is a particular principle that applies only to fermions and not to bosons, and that principle is the polyexclusion principle. Now what this principle says is that only one particle can sit in a particular quantum state at a time. Fermions have to obey this rule, bosons don't have to, and what that means is that basically if you have lots of bosons that can all sit in the same quantum state, they'll probably choose the lowest energy quantum state. Fermions, they have to kind of keep stacking up to all the quantum states available. They can't all live in that same state. So that has some consequences about how those particles act within certain systems.