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Billy Crystal made an appearance on Thursday's Late Night With Jimmy Fallon for an encore performance of Who's On First?, and it included another funny man showing up on the scene!

Jerry Seinfeld surprised the audience when he came in on the Abbott & Costello sketch, finally revealing the identity of the St. Louis Wolves' first-baseman "Who" and their second-baseman "What."

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But wait, who is Seinfeld portraying? Check out the video to find out!