I had someone ask me how rockets are able to accelerate in space and my initial answer was that the rocket fuel combusts and is heated into an energetic gas, the gas is accelerated out of the back of the rocket (i.e. the rocket exerts a force on the gas), then according to Newton's 3rd law, the gas exerts an equal and opposite force on the rocket resulting in the rocket accelerating in the opposite direction to the gas. Although a simplistic explanation I think this is correct?! The person wasn't quite satisfied with this answer and wanted more details, such as, what is the gas pushing on in the rocket (by this I think they understand that there doesn't have to be anything external to push on, but they are unsure as to how the gas pushes on the rocket). My follow-up explanation was the following: Imagine a super simplified model in which the rocket is completely hollow inside, then without any nozzle at the rear end, the pressurised gas on the inside of the rocket would exert equal forces on all internal walls of the rocket. Newton's 3rd law then states that the (rigid) walls of the rocket will exert an equal, but opposite force on the gas and everything will remain in equilibrium (since on average there will be an equal amount of force acting on the left and right walls of the rocket, and the top and bottom walls of the rocket) . Imagine now that we create a nozzle at the bottom end of the rocket, now the situation has changed. The forces exerted by the gas on the sides of the rocket will remain balanced, however, now the top end rocket wall will a force exerted on it, pushing it upwards, but there is now no balancing force acting on the bottom end, since the gas is escaping out of the nozzle. Hence there will be a net force acting on the top end of the rocket, propelling it in the opposite direction to the gas accelerating out of the nozzle at the bottom end. Would this be a valid explanation or have I made some errors (if so, could someone provide a detailed correct explanation)?!