Lior Elazary was hoping to teach his children how the CPU inside a computer actually functions. Rather than getting into the history of the transistor, the semiconducting nature of silicon, and the rise of the microprocessor, Elazary decided to demonstrate the logic behind it all. He created a clock with the essential elements of a CPU — the arithmetic processing unit, buses, RAM, register, and a control unit — and created a physical process with a ball bearing and "flip flops" that all work together to tell the time. The idea of "hard-coding" programming instructions is also tangibly demonstrated, as the programming is literally built-into the pinball action. Although you could (and some Slashdot commenters have) debate about whether the metaphors here map exactly, it's hard to debate the clock's elegance or utility as a teaching tool. You can visit the Instructables link below to learn how to build one yourself.