After my first home invasion, I went out and bought a bag of them from the dime store. I had one in every room of the house.
It was a hot night, and I'd been working on my house all day and I was lying naked on the bed when 2 guys came in. I grabbed my claw hammer out of my tool belt and backed them down the stairs. They weren't out to get me; they wanted to visit the hooker that used to live on the 3rd floor.
There are a few good Youtubes out there explaining their use. I grew up in a construction family. Dad showed me around the claw hammer and the spade shovel. He had to keep saying, "But don't do it this way, you'll kill the guy."
