I think I know why Sir Isaac Newton invented the cat door. In addition to the law of gravity, he probably had discovered that other, equally inviolable law of nature. It's the one that says a cat who is outside wants to come in, and a cat who is inside wants out.
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I once knew a man whose little shorthair, Hazel, sat for hours gazing at the backside of the front door. Hazel knew that gazing worked. Eventually, the door would open and she could go out. When she was ready to come back in, she sat on the front porch and resumed gazing from the other side.
One day, Hazel's human tried an experiment. He opened the door and let her out. A few minutes later, he opened the door and she came in. After a couple more minutes, he opened the door and she went out. A few minutes later, he opened the door and she came in. This continued for 12 repetitions, at which time the man was laughing uncontrollably and Hazel was patiently waiting to be let out again.
When my cat, Clementine, took up residence with me, I decided she'd better have a cat door. A friend obligingly took a saw to the side of my house. The door was nothing fancy, an opening cut between the studs. Clem liked it fine.
Enter the porcupine.
His bulbous hindquarters kept him from coming all the way in. His aft-pointing quills kept him from backing out. Pinned between interior and exterior walls, he smacked his tail angrily. It took a jam bar, a framing ax and a great deal of caution to dismantle enough house to set him free.
After that, I bought a cat door with a hinged flap and a battery-powered latch. A small magnet on Clementine's collar would unlock the door for her alone. The friend who'd braved the porcupine volunteered to install the new contraption in the hole.
I came back from dealing with laundry to find an indignant cat and my friend examining hands that looked as if he'd just withdrawn them from a Cuisinart. "Clem won't go through the door, he said."
Of course she wouldn't. It wasn't her idea - yet. When you install a cat door with a flap, you have to tape it open for a day or three, until the cat is confidently going in and out. Then you drop the flap. The cat will sniff and nose and paw and finally push it aside. No need for instruction. Or laceration.
Now Clem and her sidekick, Annie, have a door that locks in any of four positions. One is free-swinging, allowing access from either side. Cat-collection mode permits entry only. Cat-expulsion mode allows only egress. Lock position means no swinging, no way.
A cat door in Douglas was on "lock" last month, when Rufus, a red-haired tabby, discovered to his dismay that he was hosting a visit from the traveling vet. Rufus pussyfooted to the far end of the room. Then he whirled and made a run at the cat door. Pow! Leading with his shoulder, the cat knocked the door from its mounting and fled. "I don't believe what I just saw," the vet said.
Moral: If a serious lockdown is what you have in mind, look for a cat door with a removable, solid panel that slides down over the swinging flap.
Cat doors aren't just for exterior walls. One between the kitchen and utility room lets the cat enter while keeping the dog out of cat food or litter box.
And a big, gray cat named Dusty who lives in my neighborhood discovered a unique use for his cat door. He sits just inside with his head poking out, shielded from rain by the flap that rests like a visor atop his head.
Linda Daniel has spent her life in the company of cats, most of whom simply showed up at her door. She's a believer in spaying and neutering to reduce the number of homeless cats. E-mail: firstname.lastname@example.org.