If you've ever heard a knock-knock joke, you've experienced the often groan-inducing form of humor known as a pun. A pun is not always funny, but it is always a play on words. A pun can be made up in several different ways, based on the meaning or sonics of the words involved. Words could sound alike but have different meanings, like "jeans" and "genes," or they could be spelled the same but have two or more meanings, like the word "club." A knock-knock joke depends on the use of a pun: "Knock, Knock." "Who's there?" "Orange." "Orange who?" "Orange you going to let me in?"