Picture of Instant Caltrops
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."

Safety Warnings:
Flying sharp metal will hit you in the eye.
If someone steps on one of these they'll get hurt.

Excellent photos by theinfonaut
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Step 1: Nicaraguan Caltrop

Picture of Nicaraguan Caltrop
Caltrops in the Gallery of Heroes and Martyrs in the town of Masaya, Nicaragua.
These look like barbed wire barbs cut off and welded together, but they're much larger than any barbed wire I've seen. These were used in the revolution against Somoza in the late 70's. Nicaraguans threw them onto roads in front of troop truck convoys to delay them with flat tires.
Somoza was a ( yawn, yet another ) U.S. supported brutal dictator.

By the way, a recent issue of Time magazine reported that Nicaragua now has the lowest risk of terrorism in the hemisphere.

Step 2: Chicken Wire Ain't Chicken No More

Picture of Chicken Wire Ain't Chicken No More
Cut off some cross-shaped chunks from the chicken wire using a bolt cutter.
Then cut the ends off again at an angle to make them sharp. Watch out for flying bits of sharp metal hitting you in the eye.

Step 3: A Tripod and a Spike

Picture of A Tripod and a Spike
Bend the sharp wires as shown. You're done!

Be super careful with these nasty things and don't leave them lying around.
These are only to carry in your pouch at the renaissance festival as part of your costume.
wow, terrorism? really?