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A cat flap (also known as a doggie door or pet door) is
a hinged flap set into a door, wall or window to allow cats,
dogs, or other
domestic pets to
enter and exit a house on their own without needing a human to open the door.
They are very popular with cat owners, many of whom like to encourage their
cats' natural independence by letting them come and go at will — particularly in
the
United Kingdom where it is believed that about 90% of all cats (including
ferals)
have access to the outdoors, many of these via cat flaps ([1]).
Flaps for most dogs must be bigger and therefore are often constructed
differently; except in rural areas they tend to give access to fenced-in yards
or gardens rather than the neighborhood at large.
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 flap 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 flaps have
electromagnetic catches that can detect a matching
permanent magnet worn on the animal's collar and unlock the flap as the
animal approaches, thus preventing any other animals using the flap 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 flap 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 cat flap is often attributed to Sir
Isaac
Newton in the
seventeenth century; this story, however, is usually related ([2],
for example) without any sources to support it
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