Now, for each of those K possibilities, how many things could be in the second spot? Well, it's going to be K minus one, because you already put something in the first spot, and then over here, what's it going to be? K minus two, all the way to the last spot. There's only one thing that could be put in the last spot. So what is this thing here? K times K minus one times K minus two times K minus three, all the way down to one. Well, this is just equal to K factorial.