Backpropagation in a simple neural network.
This diagram walks through the full process:
> Forward pass with sigmoid activations σ
> Mean squared error loss E
> Backward pass computing gradients ∂E/∂z, ∂E/∂a, ∂E/∂w, and ∂E/∂b
Clearly shows how error flows backward to update weights and biases using the chain rule.