An ancient weapon like a metal sandburr. It's a nail with multiple points that has at least one point upward no matter how it lands. They are also called "star nails". This is the quickest way to make them.
Some people consider caltrops a political weapon in this era of wars for oil for vehicle fuel.
If you are such a person you should pursue your ends by peaceful means instead.
Remember Noam Chomsky's advice:
"If you fear the government the proper response is to get more involved in government, rather than less so, because the Government really is the institution that represents you."
Watch the movie "Weather Underground" to find out what some famous political vandals wish they'd done instead.
I think it was Bill Clinton who said, "It's best to conclude that bad policy is caused by ignorance, not malice."
Flying sharp metal will hit you in the eye.
If someone steps on one of these they'll get hurt.
Excellent photos by theinfonaut