Many years ago, I taught a course at the University of Aberdeen on the ‘4As’ of anthropology, archaeology, art and architecture (Ingold 2013). As we had been discussing flint-knapping, I invited the master-knapper, John Lord, to give a demonstration. We watched in awe as he skilfully detached flakes from a flint nodule of irregular shape to reveal the classical, bifacial form of an Acheulean handaxe. Then it was our turn to use wooden or antler hammers to detach flakes from fragments of flint. After an hour or two, none of us had made any headway. What looked simple in practised hands would have required years to learn, not a single afternoon! Nevertheless, the workshop taught us an important lesson. As Bentley and O'Brien (2024) remind us, mastering the skills to make an Acheulean biface requires hundreds of hours of practice. The question is, why does it need so long? What is going on during these many hours?