How do rockets work? Sometimes I teach at university, and everybody likes a live demo. A fire extinguisher is just a small rocket nozzle; a chair & cart make a good spaceship. Folks ask how it works in space with no air to push against? But it's the mass blasting out that makes me roll. Doesn't need any backstop. Newton's 2nd law. But always good to have a landing plan - thanks @UWaterloo students!