Act 04 is not an animation of training — it is training. A small network (2 → 10 → 8 → 1) is running real backpropagation in your browser, on two interleaved spirals that no straight line can ever separate. The coloured floor is that network's live output sampled on a grid, and the green curve is its loss. Reload and it learns again from scratch, slightly differently. Drag to look around.