 In the fire syringe demonstration, a small piece of nitrocellulose is put in the inner tube. A plunger is inserted in the top of the tube and traps a sample of air. The plunger is rapidly depressed and as the plunger moves down the tube, the air molecules strike the plunger and pick up energy. This increases the average kinetic energy of the gas in the tube and increases its temperature. The temperature quickly rises enough to cause the nitrocellulose to ignite.