Who's on what forum?

Abbott: Goofe' Dean. Well, let's see, we have on the bags, Who's on first, What's on second, I Don't Know is on third...
Costello: That's what I want to find out.
Abbott: I say Who's on first, What's on second, I Don't Know's on third.
Costello: Are you the manager?
Abbott: Yes.
Costello: You gonna be the coach too?
Abbott: Yes.
Costello: And you don't know the fellows' names.
Abbott: Well I should.
Costello: Well then who's on first?
Abbott: Yes.
Costello: I mean the fellow's name.
Abbott: Who.
Costello: The guy on first.
Abbott: Who.
Costello: The first baseman.
Abbott: Who.
Costello: The guy playing...
Abbott: Who is on first!
Costello: I'm asking you who's on first.
Abbott: That's the man's name.
Costello: That's who's name?
Abbott: Yes.

Who's on first?