**Teacher:** I don't know, can you borrow a pencil?
**Student:** Aha, but I clearly meant to ask for permission. Since you and the rest of the class understood my intent perfectly well, and the word "may" to show permission is rapidly falling out of fashion, there is nothing wrong with asking...
Why is it so hard to explain a pun to a kleptomaniac?
They don't understand the subtle nuances of the English language, so they can't pick up on the double entendres needed to appreciate a good pun. It may be a generalization, but in my experience that's how it works out.