Trapa natans, the water caltrop, is a widespread species of flowering plant in the genus Trapa, family Lythraceae, grown as a food crop. Other vernacular names include buffalo nut, bat nut, devil pod, ling nut, mustache nut, singhara nut, water nut, and water chestnut, though this last name is commonly applied to Eleocharis dulcis.

It is a floating annual aquatic plant, growing in slow-moving freshwater up to 5 metres (16 feet) deep, native to warm temperate parts of Eurasia and Africa. It bears horned fruits. Each fruit contains a single large seed. T. natans has been cultivated in China and the Indian subcontinent for the edible seeds for at least 3,000 years.

In feudal Japan, the fruits were used as substitutes for iron caltrops, an area denial weapon; they were sharp enough to penetrate the shoes of the period. The Japanese manufacturing group Mitsubishi takes its name and its logo from the plant. The Bengali name for the samosa or fried triangular pastry snack, shingara, is likewise the name of the plant.

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