The water caltrop (Trapa natans) is the starchy nut of a floating aquatic plant, encased in a hard, glossy-black shell drawn out into two or four sharp horns that gave it its name. Boiled and shelled, the white kernel tastes mild, sweet and starchy — like a cross between a chestnut and a water chestnut — and is a staple snack and ingredient across East and South Asia.