Cornstarch is used to thicken liquids in a variety of recipes such as sauces, gravies, pies, puddings, and stir-fries. It can be replaced with flour, arrowroot, potato starch, tapioca, and even instant mashed potato granules. The ingredient you are most likely to have on hand, of course, is flour, so we'll start with that. When you want 1 cup of liquid to be fairly thick, it takes 1 tablespoon of cornstarch.