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

Lets say I have an explosion in the middle of the vacuum of space. Stuff goes in all directions.

Now, lets say I put a shield on one side. Stuff only goes one way, the shield is blown the other way.

That shield is my rocket, but its directing its explosion in one specific direction, the rest of the rocket is being shoved the other way.