I was looking on wiki about different froms of propulsion and I have this very general question. When a rocket produces thrust from its exhaust, what exactly is it 'pushing on' ? I mean on Earth, when a jet plane uses its engines to take off, what is going on...is the exhaust 'pushing' on the particles in the atmosphere or against its own exhaust? I think the answer to the plane will satisfy a rocket in space too..so I'll wait to ask. Thanks, Casey p.s. Sorry if this is a stupid question....it just occured to me that I don't know the answer and I now need to!