These are great for medieval props!
Step 1: About
The Caltrop was used by many in the Middle Ages. When thrown, a spike always landed pointing up. It was used to slow down the enemy. It was also used in WWII. Now, they are used to deflate tires.
Make two thin foil bars
Twist in a way that they stay together and whenever thrown a spike points up.
Step 4: Done!
Don't step on it!