Simple Networks for Learning - A not so simple demonstration...

We'll start with a simple brain (or a simple computer) which consists of four simple elements connected as shown to the left. The bottom two ovals we'll call "input" units and the filled circle we'll call an "output" unit. They are actually all we really need (for present purposes), but things will work a little better if we have a fourth "bias" element, the circle with the B in it.

http://serendip.brynmawr.edu/complexity/perceptron/learning.html