An implementation of the classic Chinese abacus
There are books on how to use an abacus, but basically all it does is add and subtract, the rest you have to do in your head. Essentially, this is a proof by induction that a computer is more powerful than an abacus, since a computer program can contain an abacus. (But then again, you can simulate a computer within a computer, so what does that show). Actually, with a real abacus, one can move more than one row at a time with 10 fingers. But on the other hand, a real abacus does not have the current sum displayed as an integer.