| "Caltrop" may refer to one of two things: a military defensive weapon against tires and feet, or a type of plant that produces seeds or burrs that have spines like those of the weapon.
Defensive caltrops are tetrahedral -- which is to say, they have four points, with each separated from each of the others by an angle of 109.5°. With four spikes, any three of them may land on the ground, and the remaining one will always stick straight up. The three point support also means they are (mostly) angularly stabile, so that a tire driven over them will be more likely to be punctured than to just push the caltrop over.
Modern designs are more advanced than their medieval counterparts. The version used by the army is fashioned from a single, flat piece of metal, and has "arrowheads" cut to make each point. The arrowheads are razor sharp, sharp enough to puncture most boots with only a human's weight as pressure. The arrowhead shape makes them able to stay embedded in a tire after puncture, thus doing irreparable damage and defeating self-sealing tires. The army caltrops also have two holes in them, so they may be chained together to form a makeshift STD barricade. These are also hard and strong enough to wedge between tank treads and stop the tank from moving.
Another kind of military caltrop on the market is made of hollow spikes welded together -- think of it as four sharpened straws attached to one another. When a self-sealing tire (or any tire, for that matter) drives over these, it deflates through the spike that makes the puncture.
Ghetto caltrops can be fashioned easily, if you ever find yourself in a warzone. Cut the heads off of two nails, and grind the cut end so it is as sharp as the sharp end. Bend both nails 109.5°, or a close approximation thereof (once you've gotten it wrong a couple of times, you can probably just eyeball the angle). With one person holding each nail in tongs, and another person using the torch, weld the two nails together, forming a perfect caltrop of whatever size you need. Another method is to cut a four pointed star from a flat, heavy sheet of steel, and bend each spike into the proper position. This is not as good, because it either has to be difficult to do the bending (thick, good steel), or the caltrop will flatten when a car drives over it (thin, lousy steel). If you choose to implement either of these designs, don't forget to spraypaint the caltrop matte black, so that drivers and infantry will have a harder time seeing it.
Water caltrop is the common name for any member of the genus Trapa, all underwater plants with floating leaf structures that produce hard, spiny fruit. One of the more interesting kinds of water caltrop is the Trapa bicornis, known as the water chestnut in English and the ling kio in Mandarin. Its fruits bear an uncanny resemblance to a bull's head with spines for horns, and are commonly made into jewelry.
Also of interest is the Tribulus terrestris, known simply as the caltrop. It is a flowering summer annual which produces clusters of five burrs as fruit. The burrs are four pronged, with two long, sharp spines -- strong enough to puncture shoes and tires -- and two short ones. Caltrop is considered a weed, and is a big problem for sheep farmers. Sheep become highly photosensitive after grazing on caltrop, and will sunburn and lose their wool if not put in a shady pasture or dark shed. Caltrop grazing may also lead to nitrate poisoning (causing death in six to twelve hours) and is neurotoxic, which will leave the sheep with a stagger given enough exposure. | 
