The umbrella on a blue-sky day

Give it one: a day that stayed dry.

That morning, the sky was clear and the weather app said dry. Your friend insisted that it would pour.

With careless settings, the neuron trusts your friend just as much as it trusts the sky. The total crosses the threshold, so its answer is: carry an umbrella.

You lug it around all day under a blue sky. Now the correct answer is clear: you did not need it.

The neuron's answer and what actually happened do not match. That mismatch tells us that at least one weight needs to change. The machine still needs a way to decide which one.