A starchy substance extracted from the root of the cassava plant. Tapioca granules/flakes/pellets are used to make tapioca pudding and to thicken pie fillings. The grains don't dissolve completely when cooked, so puddings and pies thickened with them end up studded with tiny gelatinous balls. If you don't mind the balls, you can also use tapioca to thicken soups, gravies, and stews. If the balls are a problem, just pulverize the tapioca in a coffee grinder or blender, or buy tapioca starch, which is already finely ground. Tapioca tolerates prolonged cooking and freezing, and gives the fillings an attractive glossy sheen. To use it in a pie filling, mix it with the other ingredients, then let it sit for at least five minutes so that the tapioca can absorb some of the liquid. Don't confuse instant tapioca with regular tapioca, which has larger beads, or with the even larger tapioca pearls sold in Asian markets. Minute® tapioca is a well-known brand.
cornstarch (Use half as much. Cornstarch breaks down if it's mixed with acidic ingredients, cooked for a long time, or frozen and thawed.) OR arrowroot (more expensive) OR flour (Use a little more.)