- The fruit of an aquatic plant, Trapa bicornis, with a shiny brown to black skin and two projecting horns. It has been used for food since neolithic times and is still grown and eaten in China, Korea and Japan. It has a semi-sweet potato-like flesh and must be boiled for an hour to destroy parasites. It is used in various dishes and preserved in honey or syrup. Unfortunately it is often misnamed water chestnut and a flour made from it is called water chestnut starch.