Backpropagation: The Secret to Teaching Networks What They Did Wrong

We've talked about how neural networks make predictions through forward propagation, and how gradient descent helps them learn by adjusting weights. But we've been glossing over a crucial detail - how does the network actually know which weights to adjust and by how much? That's where backpropagation comes in - one of the most elegant algorithms in computer science and the secret sauce that makes deep learning possible.