[jdietrich]

Machine code on an 8-bit processor - ideally an old microcomputer, failing that an ATtiny. It's the only sensible way of teaching the fundamentals of how computers actually work. A reasonably bright young teenager can easily understand a computer from the metal up, so long as that computer has a bare-bones instruction set and no more memory than can be copied onto a couple of sheets of paper. There are a number of excellent books written for the Vic-20 or Timex 1000 that cover all the fundamentals in an accessible way.