A high-tech cat door called SureFlap works like a virtual bouncer. The battery-operated door can be programmed to read your pet’s microchip, denying access to unapproved guests. Physicist Nick Hill created the device so that his cat, Flipper, could enjoy the great outdoors, then return home without a gaggle of freeloading feline friends in tow. It can also be used to more ingenious ends, such as helping your cat stay on a diet.