Some jokes about modal verb can

An American professor was having a class, and a student asked, "May I take a bath?"

The professor replied, "I don't know. You must know if you can take a shower. "

What the students here want to express is obvious, that is, can I go to the toilet? Of course, the professor knows what he means, but he is just teasing him. . It can also mean "people have certain abilities", such as the ability to go to the toilet. So the professor said, "I don't know (whether you have this ability), you need to know whether you have the ability to go to the toilet."

Actually, it is impolite to ask people to use can. Better say I can or do you mind ... and so on. This is why the professor played a joke on him.

That's probably what it means. I don't know if it's clear.