My grandmother, Sarah Tillman, has been making this type of casserole since the 1940s. She's tinkered with it over the years, but she's confident that sometime in the '50s—probably the first time her in-laws came to dinner—she got the mix right. She starts with fresh, not canned, sweet potatoes. The kind she favors, which are about the size of baking potatoes, have smooth skin and either white or deep orange flesh. Yams, to set the record straight, are monocotyledons (plants that sprout from a single-leaved embryo) that originated in Africa or Asia. They come in many shapes, and most have a hairy, barklike coating and white or yellow flesh. Sweet potatoes are dicotyledons (from a two-leaved embryo), native to Central America, and often sweeter than yams.