Who’s on first, What’s on second, I Don’t Know is on third.

The line comes from Abbott and Costello’s famous “Who’s on First?” routine, a fast-paced baseball sketch built on misunderstandings over players whose names are ordinary interrogatives (“Who,” “What,” “I Don’t Know”). Lou Costello plays the exasperated straight man trying to learn the lineup, while Bud Abbott calmly answers with the players’ “names,” escalating the confusion. The routine was developed in their vaudeville act and became nationally popular through radio and film appearances in the early 1940s, cementing Abbott and Costello as major American comedians and making the sketch a touchstone of wordplay-based comedy.

On the surface, the sentence is a simple roster: first base is “Who,” second is “What,” and third is “I Don’t Know.” Its comic force comes from collapsing two functions of language—asking a question and giving an answer—into the same words. Costello’s frustration mirrors the audience’s awareness that the “answers” are technically correct yet pragmatically useless, because they refuse to resolve the conversational intent. The line showcases how comedy can arise from semantic ambiguity and conversational misalignment, and it has endured as a shorthand for circular confusion, miscommunication, and the limits of literalism.

1) “Who’s on first, What’s on second, I Don’t Know’s on third.”
2) “Who is on first; What is on second; I Don’t Know is on third.”
3) “Who’s on first, What’s on second, and I Don’t Know’s on third.”