 FACT: When the gas is ignited from the cylinder in the doctor’s  office, the cylinder goes shooting down the hall. You can see that  the valve is still intact. In reality, the gas escaping from the  cylinder through a valve which is at a ninety degree angle to the  cylinder such as the one shown, would force the bottle to spin, not  to shoot out and fly in a straight line. Additionally, if the  pressure of the gas were enough to move the cylinder, it would do so  regardless of the gas being lit on fire. The burning, and  subsequently expanding, gas would have no effect on the cylinder  once it is outside of the valve as the burning gas does not increase  the force on the cylinder.