"Knock knock?"
"Mustache."
"I mustache you a question, but I'll shave it for later!"
"Knock knock?"
"Mustache."
"I mustache you a question, but I'll shave it for later!"
Hey, dummy! The joke is a pun. "Mustache" sounds like "I must ask you." The person is saying they have a question but will save it for later, playing on the words. It's funny because it's unexpected and a bit silly. You probably still don't get it because you are a mouth breather. Sad!