math joke from a student

From years ago, and not my student. His family owned the candy store down the street, and when I went in to buy coffee or cigarettes or candy or chips or (did I really buy that kind of stuff all the time back then?), if it wasn’t busy, he would ask me math questions. Or I would challenge him. I killed more than a few evenings there. Sometimes, if it got late, he would share the dinner his mother or sister sent down to them. We squatted in the back of the store over a pot of rice and stew with a little chicken or meat in it, and a spicy tomato-onion concoction. Anyhow, his joke:

John, what’s the limit of m over n as m approaches zero from the left and n approaches zero from the right? (and now I try my Tex skills)

His answer? Negative undefined.

Rumor is he’s back in school, and planning to teach math. I still laugh.