A force is simply a push or a pull. Forces are needed to change the motion of objects – even tiny particles of air. Air is a gas made of a mixture of molecules. Like other gases, air particles expand to fill any container that they occupy. An air cannon works by applying a force quickly to the air particles contained within the cannon. When you push in on the sides of the box, the sides collide with air particles and push them toward the opening of the cannon. These air particles collide with other air particles setting off a kind of chain reaction. The only way for these fast moving air particles to escape is through the opening. The rapid movement of these particles forms a stream of air that flows quickly out of the cannon and across the room. How far away can you feel the puff?