Linked from the title above: a page describing the construction of logic gates (the fundamental building blocks of all modern computers and digital circuitry) from Lego. It uses a lot of "unusual" components, such as rack gears, which I assume must be special ordered from the company (at least, my kids' Legos don't have them, and I don't recall seeing them in the toy stores). The builder has NOT, OR, NOR, AND and NAND gates, which is more than sufficient to implement any logic function. To build a reasonable computer model, one would also need a tri-state device, in which one input disconnects or connects the other to the output. I don't think that would be a problem to implement in Lego. Of course, the other problem in building a complete Lego computer would be the increase in force needed at the input as more and more gates are added --- eventually, the force would be greater than what could be applied at the input without popping it apart. To get around that, you might be able to make some of the gates powered by motors and couple those gates via switches (rather than mechanically). Oh, and you'd need to spend a lot of money.