JB:  My son and I were building a puzzle just the other day and I was explaining this distinction to him. Definitely the edge pieces first. But my strategy is never to put all the edge pieces down and then fill in the rest. I tend to build in sections, let the shape start to take form, use the green of tree leaves to teach me where the brown bark should be and so on. I’m no expert, though. My son is five. We build really simple puzzles.