 Caltrop with hollow spikes to puncture self-sealing rubber tires  Contemporary caltrop improvised from large nails welded together. A caltrop (jack rock, star nail) is a device made up of four (or more) sharp nails or spines that are arranged in such a manner that one of them is always pointing upward from a stable base (eg, a tetrahedron or tetrapod). In the military context, it has been used to slow down the advance of horses, war elephants, wheeled vehicles, and human troops.
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.
Other usage
In role-playing games, 4-sided dice are referred to as "caltrops" because of their size and shape and the tendency of dice to fall and roll underfoot.
Students often play practical jokes by twisting staples together to form a mini-caltrop, then placing it on a hard seat where the victim will sit on it. These are often referred to as "ass scorpions"
Environmental activists
Use within the United States against logging truck tires in the 1970s were quickly condemned by Earth First! due to the possibility of such use constituting a health and safety hazard to both humans and other animals.
The Earth First! committment toward nonviolent resistance includes the non-use of caltrops and traditionally other direct action organizations and individuals have adopted the non-use of such devices.
See also
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