Gunpowder
It burns very rapidly, and creates gases. Those gases use up more space than the gunpowder they come from, so they push outward. If the gunpowder is in a small space, the gasses will push on the walls of the space, building up pressure. A bullet  is pushed on in this way inside of a gun, causing it to fly out at high speeds. Gun powder is packed into metal cylinders behind the bullet, called the cartridge.