We developed this as a two object system (ship and fuel/gas). How would you explain a rocket ship to someone who has seem a model rocket, bottle rocket or fireworks shot into the air and woners how that works. Did you ever blow up a balloon and just let it go? Okay...I know that 2 objects has to do with Newtons 3rd Law as it is the only one that contains 2 objects. And I think that the gas or fire or whatever pushes against the earth. And the earth pushes against them also. Then I'm not sure.