Venus's Aquarium

A seemingly regular aquarium, populated with dwarf lanternsharks and/or any other carnivorous fish (piranha?) that will eat mammals if they are made available to it. The top of the aquarium is slotted like a kitchen spoon, with an oscillating fan atop it to blow the aquatic scents far and wide. The slotted lid, accessible to even the craftiest feline only by leaping atop the aquarium, is in fact a trapdoor held shut with a 5-pound-test spring.

Placed in an open window to serve as both decoration and lawn/garden pest control, it is no more cruel to cats than Venus's Flytraps are to flies. Don't want to see your own cat end up as fishfood? I recommend keeping your cat out of the yards of such homeowners, and in return we will gladly continue to control our own pets thusly.

If it wasn't so bloody impossible to catproof a lawn, such measures wouldn't be necessary. Here in Spokane, Washington, USA, the maximum legal height for a front-yard fence or wall is 4 feet (6 feet for the backyard) and topping it with barbed wire, nails or broken glass to keep small animals from landing atop the wall or fence is illegal.

A 4 foot fence is more than adequate for keeping stray dogs out (and usually adequate for keeping them in, too). They actually respect well-defined boundaries.