So for example, in all of this is review, if I were to say three factorial, that's going to be three times two times one. If I were to say two factorial, that's going to be two times one. One factorial, by that logic, well, I just keep decrementing until I get to one, but I don't even have to decrement here. I'm already at one, so I just multiply it one. Now what about zero factorial? This is interesting. Zero factorial.