Best little pressure experiment (Full Experiment Linked) 

Check out how decreasing the volume and increasing the pressure can lead to combustion in this fantastic experiment. By placing a flammable material like cotton in the bottom of a clear piston (the fire syringe aka fire syringe) and slamming the rod down quickly we can elevate the internal temperature enough to initiate a combustion reaction of the cotton with the oxygen in the piston. It is super cool. Normally fire starts with something hot, but in this reaction we see we can add the necessary energy in other unlikely ways.