Sorry if this is the wrong place to ask this, but I feel it's a question of vital importance to the future of math education.
I hear this listed as a "math joke" all the time and I've never got it. I've always thought that it was just dumb, but have started to wonder if I'm missing something. Is the joke literally that it's just unexpected to take $\varepsilon < 0$ since we usually let $\varepsilon$ represent a small positive number, or is there something else to it?