I first saw this joke on bash.org twenty or thirty years ago and it's been my go to ever since. I had to fight to resist posting it until at least the week of Halloween.

A fun anecdote about this joke, I once interviewed at NASA. It was a hard choice, but I ended up turning down the job. A few months later, my wife and I went to a birthing class and ended up chatting with the couple in front of us. Turns out the guy worked at NASA and actually knew the person who interviewed me. After getting an understanding of his role, I told him this joke.

He laughed. His wife turned to mine and said "what are they laughing about?" My wife said "I don't know. Math, I think."