Because it's a joke about the number of letters in couch. Five letters and if you take away one that is equal to 20%. So he said "ouch". I like that part. It made it funny. Like who would steal 20% of a couch. That's just this jokester being a silly goose. Because no one would steal 20% of a couch. But if they did, that wouldn't be funny. It's funny because of how couch is spelled. With five letters and if you take 20% of the letters you can spell "ouch" which is the funny part. Then I like the puns people add in. Like with plays on the word couch. Those are the real knee slappers. You people are just too much to handle. I bet they were the class clowns.