A paper popper is a partyprank that is commonly practiced in public schools. There are many variations of a paper popper, but they all involve a folded sheet of paper being gripped and thrust down. This causes air to be forced into the paper's folded flaps, making the paper flaps pop out in the opposite direction at a high velocity, thus making a loud popping noise. According to popular misconception the sound is supposed to be caused by the paper breaking the sound barrier. What actually happens is that air rushes in a blast wave to fill the vacuum created in the pocket that opens. The sound is described as the crack of a whip, which is an example of breaking the sound barrier, although the causes are different. Most papers work for poppers, although some are louder than others.