"Who's on First?" is a comedy routine that was performed most famously by the comedy duo, Abbott and Costello, starting in the late 1930's. Its absurdist humor rests on the players of a baseball team having last names that are common words or phrases in English: the first baseman is "Who"; the second baseman is "What"; the third baseman is "I Don't Know"; etc.
In the United States, "Who's on First?" has become an iconic piece of humor with dozens of variations created over the past several decades. Not only was it named the Best Comedy Routine of the Century by Time Magazine in 1999, but it also has been embraced by the Baseball Hall of Fame.