If you don't get it, look here:
YouTube - Who's on first?
TS = Twilight Sparkle, PP = Pinkie Pie, AJ = Applejack, F = Fluttershy, RD = Rainbow Dash
TS: Hooves on first.
PP: Hooves on first?
TS: Yes, hooves on first.
PP: Whose hooves on first?
TS: My hooves on first.
PP: MY hooves on first?
TS...