No, not what you're probably imagining.
A cat, named Flo, kept bringing live animals in through the cat door, to play with and chase through the house.
So, these entrepreneurs, decided to rig up a box to the outside of their cat door, and only let dear old Flo in if she wasn't carrying something.
How'd they do it? Read all about it.
Courtesy of User Friendly's LOTD.