Who's On First?
"Who's on First?" is a comedy routine made most famous by Abbott and Costello. In Abbott and Costello's version, the premise of the routine is that Abbott is identifying the players on a baseball team to Costello, but their names and nicknames can be interpreted as non-responsive answers to Costello's questions. In this context, the first baseman is named "Who"; thus, the utterance "Who's on first" is ambiguous between the question ("which person is the first baseman?") and the answer ("The name of the first baseman is 'Who'").
Read more about Who's On First?: History, Sketch, Writing Credit, Notable Performances and Derivatives, Real-life Parallels