The number of functions from the empty set to the empty set

I cannot read this post without posting something in response. First, I wasn't aware that I act like a mathematician, even when I am not doing mathematics. Second, the only math joke I know[1].

A biologist, a physicist, and a mathematician walk past a house. As they are walking past, they notice that two people walk in, but later those two people walk out with one additional person. The biologist says, "They must be multiplying." The physicist says, "It's an anomoly." At this point, another person walks into the house. The mathemitians sees this and says, "Now there is nobody in the house."

[1] Really, it's better that we don't know that many math jokes. It saves us from embarrassment.