The first guns were basically miniature cannons. A Cannon is just a tube with an open and a closed end. To make a cannon shoot you need to have an open hole at the top of the closed end. You fill the bottom of it with gun powder and then you stick in the cannon balls. You put a string in the hole and then you light it. When the gun powder burns it expands and creates a lot of hot gas. when the hot gas expands it has to expand somewhere so it hurls the cannon ball down the barrel.