Fats and oils are members of a large chemical family called the lipids, a term that comes from the Greek for “fat.” Fats and oils are invaluable in the kitchen: they provide flavor and a pleasurable and persistent smoothness; they tenderize many foods by permeating and weakening their structure; they’re a cooking medium that allows us to heat foods well above the boiling point of water, thus drying out the food surface to produce a crisp texture and rich flavor. Many of these qualities reflect a basic property of the lipids: they are chemically unlike water, and largely incompatible with it. And thanks to this quality, they have played an essential role in the function of all living cells from the very beginnings of life. Because they don’t mix with water, lipids are well suited to the job of forming boundaries—membranes— between watery cells. This function is performed mainly by phospholipids similar to lecithin, molecules that cooks also use to form membranes around tiny oil droplets. Fats and oils themselves are created and stored by animals and plants as a concentrated, compact form of chemical energy, packing twice the calories as the same weight of either sugar or starch.