Cat flap
A cat flap is a hinged flap set into a door or a window to allow a cat to enter and exit a house on its own without needing a human to open the door.
The simplest cat flaps are just a weighted flap, but these can have problems with wind blowing open the flap or making a rattling noise. Typically a cat flap will also have adjustable catches to restrict the opening of the flap in one direction or the other — for example to allow the cat to come in, but not go out again. More sophisticated cat flaps have electromagnetic catches that can detect a matching permanent magnet worn on the cat's collar and unlock the flap as the cat approaches, thus preventing any other animals using the cat flap to enter the house.
Ideally, a cat flap should be set at a height so that the bottom of the opening is just a little lower than the belly of the cat when standing. Most cats prefer to use a cat flap with a transparent flap, so that they can be sure there is nothing threatening outside before leaving the safety of the home.
The cat flap was invented by Sir Isaac Newton in the seventeenth century.