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Caltrops come in a wide range of sizes, from small caltrops only a few centimeters across that were used by ninjas to deter humans in soft-soled shoes to massive steel (Czech hedgehogs) or concrete devices ( dragon's teeth) designed to wedge into tank treads. Scattered around an encampment at night small, sharp caltrops can cause attackers stealthily treading on one to raise their own alarm.
Modern police and security forces often use spike strips against vehicles with inflated rubber tires, a device similar to a patch of caltrops in principle.
The device is named for its resemblance to the spiked seed case of the caltrop, Tribulus terrestris ( Zygophyllaceae), which can also injure feet and puncture tyres. Cf. also to the Star thistle, Centaurea calcitrapa, whose latin name calcitrapa means foot trap.