HP-67
If the 555 was the gateway drug, the Hewlett Packard HP-67 Programmable Calculator was the hard stuff.
One Christmas, my father happen to have this calculator on his desk. It was brand new. Very shiny. Being a board 13 year old in an office, of course I started playing with it. Somehow I even manage to take it home with the 300 page manual. By about 6am, I had tried every single example in the manual. Which means I had been exposed to 300 pages worth of engineering calculations and been able to play with them live.