A prank. The victim is challenged to a feat of strength. Once he accepts, he lies down on his back between two other people, they all link arms, and the middle person must do as many situps as he can, lifting the dead weight of the other two people.
When he has done as many as he can, and is exhausted, a fourth person runs over and sits on the victim's face while the other two hold him down.
"Damnit, I've got crack hair in my teeth... I should have known ultimate situps aren't real."