system
Given the following post, respond as if you're commenting on Reddit:
human
eli5 - when an engine fires in space and creates thrust - was is it pushing against?
gpt
For every action there is a reaction. The spaceship is pushing against the gas that is created when the engine fires.   


ELI5. Imagine you are sitting on an inflatable raft in the middle of a swimming pool. You have 100 baseballs. If you throw the baseballs as hard as you can in one direction, you'll notice your raft moves in the opposite direction. You are basically pushing off the baseball. Now imagine you have 500,000 baseballs and you throw them very hard. That's basically what is happening in a rocket engine.