A pop-gun pops from an air explosion. The front end of the gun is made air-tight with a cork or plug. At the other end is the plunger, that fits the round bore just tight enough so it can be forced down to the plug. The bore is, of course, full of air. This air has no chance to get out. So when you push the plunger, the air is squeezed into a smaller space before it. It really becomes compressed air. Air doesn't like being squeezed any more than you do. To get out, it pushes and pushes until the plug gives way. "Pop!" The air instantly expands and sets sound waves in motion. As the act is quick and sharp and short, the sound made by it is also. "Pop!" just describes it. Shut your lips to say "pop." Fill your mouth with air from the lungs. Then let go and speak the word suddenly. In that way you make the same kind of an air explosion as the pop-gun. The power of compressed air is so great that it is used to make drills bore holes in the rocky walls of mines, and to hammer rivets in the iron work of bridges and sky-scrapers. Your pop gun is really a very scientific toy.