My parents rarely got me kits, but I picked up a decent assortment of second hand pieces over the years including a fair bit of Technic. I had all the parts of a differential and after some trial and error I worked out how they went together. I also figured out how to build a three speed gearbox driven by an electric motor, though it didn't have a clutch so changing gears while in motion was a bit hairy! Don't think I ever worked out how to get multiple forward ratios and reverse controllable from a single stick.