eli5: If space is a vacuum, how can rockets work? What are the thrusters pushing *against* if there is nothing out there?

Question

I’ve never really understood the physics of this. Obviously it works somehow — I’m not a moonlanding denier or anything — but my (admittedly primitive) brain continues to insist that a rocket thruster needs something to push *against* in order to work.

So what is it pushing against if space is essentially a void?

In: 7157

Answer 1

Stand on a skateboard and throw something. You move backwards a little bit in the opposite direction. The faster you throw and the heavier the thing you throw the more you will move.

Rockets are like the skateboard. Rocket fuel is the thing being thrown. The explosion in the back is throwing a lot of fuel out the back of the rocket very fast, which means the rocket moves very fast in the opposite direction.