Let’s Build a Brain

Wed, 21 Oct 2026 12:00:00 GMT

The world is full of surprising behavior: Beavers building dams, songbirds singing, spiders building webs, and you reading this article.

All of these behaviors are orchestrated by neurons firing in the brain. But how exactly do collections of neurons create these complex behaviors?

This is a huge question that researchers are still working on. But over the last half century, neuroscientists have solved some pieces of this puzzle.

In this article, we’ll build a tiny creature called a spark, and we’ll give it a very simple brain. As you’ll see, even this tiny brain can create rich and unexpected behaviors.

Light eaters

Suppose this spark feeds on light. Its goal in life is to reach the light.

The light-hungry spark needs a way to sense the world. So we’ll give it a rudimentary eye — a single neuron that detects brightness and converts it into electrical pulses.

It also needs a way to move. So we’ll give it another neuron that can wiggle a flipper.

By wiring the first sensory neuron to the second motor neuron, we’ve just assembled our first neural circuit:

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