My friend Mikala has a great puppy dog Gilbert and while she was gone I was babysitting (dogsitting) for her. He is a little Boston terrier and the cutest thing ever. If I got a dog I would probably get fluffy furry kind of dog though I am more into the fuzzy kind. But anyway, Gilbert has a lot of energy and is so hyper. He likes to play all the time if possible. So, sometimes after the 500th time you have taken him out in the backyard to play ball, you just let him go through the doggie door and get some stuff done or collapse on the couch. The doggie door does let cold air in the house in the winter, but other than that its a godsend for people like Mikala who have really hyper dogs. They can just let them go run around and chase the birds in the fenced in yard and the digs come back in through the dog doors when they are tired out. They do track in mud though if it is muddy out and raining. Then you have to catch Gilbert and wipe off his paws befor he gets dirt all over the house. Anyway, in the UK they call these doors cat flaps, but here in the U.S. we love dog doors because we keep our cats inside and let the dogs out in the fenced in yard because we are not abel to keep up with the energy of our pets.