Lettuce (Lactuca sativa) is the world's salad leaf — a fast-growing annual of the daisy family grown for its crisp, watery, mild-tasting leaves. About 95% water and very low in calories, it is eaten raw above all else: piled into salads, layered in sandwiches, shredded into tacos and wrapped around fillings, and only occasionally cooked. Bred over millennia from a bitter wild weed into something sweet and succulent, it comes in four broad types — crisphead (iceberg), romaine or cos, butterhead, and loose-leaf — that between them cover the spectrum from glassy crunch to soft, buttery tenderness.