I'd go with B. Kliban's "Mysterious little creatures, aren't they?" except that it doesn't make sense without the cartoon, and I can't find a link. In the cartoon, a man says the line while contemplating a cat sitting inscrutably on the floor, not realizing that two other cats are frolicking behind him on the back of the couch and making faces.
Years ago in Zimbabwe, it was common for the locals to "earn a crust" by "looking after your car" while it was parked. The theory was that they would feed your parking meter and prevent any thieves from breaking into or making off with the car. One of these guys was a character called George who had a gammy leg and used to hobble around. I asked him one day: "George, you can't walk very fast, how are you going to stop thieves taking my car?" He fixed me with a knowing look. "Sir" he said, "There is more than one way to kill a cat!"