Knock Knock / Who’s there? / Orange
Orange you going to answer the door?
This joke follows the normal knock knock joke conventions. The reason why it is funny is the fact that Orange is being punned with the word "Aren't", as in "Arent you going to answer the door".
Although knock knock jokes are not known for being the highest form of humor, this one is still pretty bad - Why would an orange be knocking on the door, anyway?