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

Like you are five?
Sit on a swing with a 10kg rock in your lap.
Now toss the rock as hard and fast as you can, and you will swing backwards. Not because you are pushing against air, but because you are pushing against the rock.

In a spacecraft, the push is because the exhaust has mass and is thrown out back really really fast.