Reminds me. A burglary attempt in my great-grandmother's house was foiled by cacti. The burglar had the misfortune to climb in the window that had a table full of her potted cactus collection under it. He made so much noise that everybody woke up, and he went right back outside again. We still have one of the original guard cacti from that incident. Long-lived thing, it's probably over a hundred years old now.

Feng shui says that spiky plants such as cacti have dangerous chi and shouldn't be allowed in a home, but this anecdote demonstrates that dangerous chi is useful for protection.