A short skit Abbott and Costello performed. While it is fun to read, it is funnier to hear (which is why I'm not posting it - find it out on the 'net, and read/listen to it).
The premise is that one of them is listening to a baseball game, and the other joins him. Not knowing who is playing, the second person asks the first about the locations of various players. What he doesn't know is that the players have names such as 'Who', 'What', and other such words. This leads to conversations such as:
#2: Who's on first?
Which leads to a confusing, but memorable dialogue which they kept going for several minutes (ie, it wasn't a one line gag, and it is hilarious to actually listen to)