To tell one of your friends to try and do the hardest sit up. Place a towel over there eyes and hold it tight to the ground. While there struggling to get up have another person pull down there pants exposing there ass. The ass will be over the person doing the sit-up's face. Pull the towel away and the sit-up persons face should shoot up straight into the buthole of the person.

When you get a person to do a blinded folded sit-up and then when they've become use to it and they trust that your not going to do anthting, you pull down your pants and when they come up, but you ass is their face.