Stewart Coffin clears a space on his dining room table and sets down a small rectangular wooden tray. Beside it he lays four wooden tiles, two triangles and two trapezoids. The object of the puzzle is to fit the four tiles within the rim of the tray. If you’re like most people, you start by fitting a square corner of a tile into a square corner of the tray. Coffin, 77, leans forward in his chair, amused. It’s a trap. “Your first mistake is putting a square corner into a square corner,” he says. “As long as you do that you’ll never solve this puzzle.”