So this is the first spot, the second spot, third spot, and then you're going to go all the way to the Kth spot. Well, for the first spot, there could be K possibilities. There's K things that could take the first spot. 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.