A boy deliberately asked, "What is menstruation?" How to answer him?

"The place that passes every month is called menstruation."

If he is a hooligan, just ignore him after answering.

If he likes you, you should let him do what you ask in the future, and don't ask some wrong questions in public to avoid making a fool of yourself.

If he is ignorant and curious, I suggest that he read more books and check himself online. Don't ask others such questions!