A dab of sunscreen about the size of a pea, a few quick strokes across the cheeks and forehead, and you are done in ten seconds. Nothing feels greasy, the bottle says SPF50, and you head out. Then a long lunch at an outdoor table leaves the top of your nose pink and the rims of your ears warm. The usual response is to blame the sunscreen and buy the next one for a bigger number. It rarely changes anything, because the number was fine all along. The pea is what went wrong. Nearly everything else...