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pet door
A pet door is a hinged flap set into a door or a window to allow dogs, cats, or other domestic animals to enter and exit a house on their own without needing a human to open the door.
The simplest are just weighted flaps, but these can have problems with wind blowing open the flap or making a rattling noise. A magnet on the flap addresses this issue by holding the door in place when not in use, but not so securely as to prevent access. Sometimes a pet door will also have adjustable catches to restrict the opening of the flap in one direction or the other — for example to allow the animal to come in, but not go out again. More sophisticated pet doors have electromagnetic catches that can detect a matching permanent magnet worn on the animal's collar and unlock the pet door as the animal approaches, thus preventing any other animals using the pet door to enter the house.
Devotees suggest a pet door should be set at a height so that the bottom of the opening is just a little lower than the belly of the animal when standing. Many animals prefer to use a pet door with a transparent flap, so that they can be sure there is nothing threatening outside before leaving the safety of the home.
The invention of the pet door is often attributed to Sir Isaac Newton in the seventeenth century; this story, however, is usually related ([1], for example) without any sources to support it.
