So they can define what it does. And mathematicians have found it far more useful to define zero factorial as something else. To define zero factorial as, and there's a little bit of a drum roll here, they believe zero factorial should be one. And I know, based on the reasoning, the conceptual reasoning of this, this doesn't make any sense. But since we've already been exposed a little bit to permutations, I'll show you why this is a useful concept, especially in the world of permutations and combinations, which is, frankly, where factorial shows up the most. I would say that most of the cases that I've ever seen, factorial in anything, has been in the situations of permutations and combinations. And in a few other things, but mainly permutations and combinations.