I've remembered in bits and pieces a thought experiment that I can't get a grasp on what was suppose to be explained and I was wondering if anyone happens to know what this is. Supposedly this was a famous thought experiment or osmething and I must have been told of it years ago, possibly even in high school but all I can remember is bits and pieces and I certainly don't remember what the goal of the experiment was. The experiment went something like this. A fly is traveling down a set of train tracks headlong into a speeding train. The thought experiment centers on what happens to the momentum of the train upon impact. I don't think it mattered whether the fly bounced off or was more realistically, "splatted" onto the train. One thing I remember was the false argument that the train would have to come to an instantaneous velocity of 0 in order for that fly to ever have 0 instantaneous velocity (since going from the positive velocity the fly original has to the negative velocity it has upon bounce or splat). Does anyone remember the entirety of this thought experiment? I'm thinking it might have had something to do with the idea of there being no such thing as a perfectly incompressible material.