Look for small to medium-sized sweet potatoes, which are sweet and creamy. (The larger ones tend to be starchier.) The skin should be firm, smooth, and even-toned; the deeper the color of the potato, in general, the richer it is in the antioxidant beta-carotene. While orange sweet potatoes are sometimes labeled yams in the United States, true yams are starchy, white-fleshed tubers common to tropical countries—not your grandmother’s famed candied casserole. Keep sweet potatoes in a cool, dry place—think pantry, not refrigerator—for up to 2 weeks. Hang on to them longer and their high sugar content will cause them to spoil.