Knock knock \ Who’s there? \ Moustache!
I moustache you a question, but I’ll shave it for later.
This is another example of a fairly normal knock-knock joke. This joke adds an extra pun to the punch line and isn't as lame as many knock-knock jokes.
The reason why this joke is funny is the puns in the punch line. The first one is saying "moustache" where "must ask" is what is actually meant, and the second one is saying "shave" where save is actually meant.
Now, in saying that it isn't as lame as many knock-knock jokes, I am not indicating that I like it - just that it isn't as bad as they get. I'll do a really awful knock-knock joke tomorrow and show you what I am talking about.