 Crushed glass. If you will crush glass and put it in the highway that they're running on, when they run over it, they cut themselves. Used it multiple times? I have used it a couple times. It was a pastor friend of mine that recommended it, and it did work. I'll be darned. Well, I'll keep that in mind. Right now I have none, and I'm very happy. But I'll keep that in mind. I'll give that a try.