Why can't your nose be 12 inches long?
Q: Why can't your nose be 12 inches long?
A: Because then it would be a foot!
This joke is a pun based on the fact that a 'foot' is both 12 inches and a body part. I suspect that this joke wouldn't work in other places where they use the metric system for measurements, but perhaps it still would.
This joke is supposed to be funny because the person who is hearing it would hear both forms of the answer at the same time.