Backpropagation Demystified: Neural Nets from...

Every modern deep learning framework — PyTorch, TensorFlow, JAX — does one thing brilliantly: it computes gradients for you. Call loss.backward() and millions of parameters update simultaneously. But what's actually happening under the hood? Backpropagation is just the chain rule applied systematically through a computational graph. By the end of this post, you'll build a neural network from scratch — no frameworks, no autograd — and understand exactly how every weight gets updated. We'll start