I've been turning over this question with friends recently: if you travel back in time, and can bring only your own body and memories--no raw materials, no machines, no books, etc., what ideas do you bring? For instance, I could probably draw and explain how to fabricate every part of a fixed gear bicycle, or at least a velocipede. But that only works in a time which has (at a minimum!) access to reasonably efficient axle bearings, and ideally rubberized tires, gears, etc. The window is small!