A neuron, at its simplest

Before the machines, start with the real thing.

A neuron is a single brain cell. You have about eighty-six billion of them. On its own, one does something almost laughably simple.

It waits for a signal. If a signal comes in, it : it sends a signal of its own onward. If nothing comes in, it stays quiet.

That's it. One input, one decision, one output. Turn the input on and watch.