Car engines and rocket engines both work by combustion. Combustion in a Ferrari car motor is different from combustion in a rocket engine. Car combustion is gas burning and exploding in cylinders called pistons again and again to push them up and down, making the car move. Rocket combustion is made by rocket fuel exploding out of the back of the rocket moving it up. The burning of most substances is bad for the environment and the ozone layer because it can let off greenhouse gases such as carbon dioxide. Hydrogen burns cleanly and gives off a lot of heat, but it does not produce much power, and is highly explosive.