a student asks you why our noses run and our feet smell.
My answer would be
Sorry, I don't know the answer to your question. Please ask the experts.
This is the professional answer I'll provide. Professional in the sense that I only say what I know.
Be careful if you try to use some sense of humor to answer those questions. You can easily step out of boundry. In particular, the two examples you have may have something to do with health/privacy issues.
If the student keeps asking this kind of questions, you'll know he has other reasons to do so (such as another student has running nose or came to classroom with bare foot). Talk to the student privately or take it to your superiors.
I received comments about part of my answer "Please ask the experts". I'll explain. I am recommending to pretend the trolling question is a serious one. I cannot guess what trolling questions the OP was asked. There are many possibilities. Many trolling questions are of personal type. I give an example here. A young male handsome prof. teaches in a college. He is asked all kinds of questions every new semester. The most frequent one is "Professor, when are you going to get married?" The prof. always replied "Sorry, I don't know the answer. You need to ask my fiancee." In this case, the expert is his fiancee. You can use other serious ways to answer the trolling question. The point is to be serious (or jokingly serious if you can) about it so that the troller won't be able to continue the trolling.
End of edit