FARGO — The classic anatomy of a dog seems like that of any other mammal. Skin. Fur. Muscle. Bone. Tendons. If Pomeranian, a garbanzo-bean-sized bladder.

But spend any time with a canine and you will soon learn that its physical presence is deceptive. Although cats — with their sinuous movements and ability to squeeze themselves into olive jars — have a well-deserved reputation as shape- shifters, I have found that dogs have similar qualities.

In fact, I have gone so far as to coin a word for dogs who can make themselves larger, tinier, skinnier or fatter than they really are, that they have full-fledged canine-plasticity.

How else do you explain a 4-pound dog who can make himself heavier than a yacht anchor when he realizes you're taking him to get his nails clipped? Or the corgi who whines that you've placed his toy too high to reach, yet can magically reach the kitchen island and eat most of the charcuterie plate? Not to mention the Great Dane who finds a way to nimbly fold himself into your Yorkie's bed or the Chihuahua who wards off bypassers by hurling himself against the privacy fence with such force that you swear a full-grown bullmastiff lives there.

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Most of the time — especially around dinnertime — dogs are solid. At other times, like when they're trying to sneak through a gap in the fence or a cracked door, they flow like liquid. And when they are in trouble, they magically transform into a colorless gas.

My dog, Wally, is a perfect illustration of this. He walks beautifully on a leash — unless he sees a squirrel. Within seconds, he magically shape-shifts from 13 pounds to 3 pounds, until I'm standing there, holding an empty collar at the end of a leash.

He performs equally miraculous feats when getting weighed at the vet. Although the platform to the scale is large enough to accommodate a Newfoundland, Wally 's atoms somehow disperse in a gravylike manner so his whole body is never all on the scale at the same time. (Granted, I understand this: If I could, I would perform similar shape-shifting voodoo whenever the nurse tries to weigh me at the doctor's office.)

When measuring him for his Halloween stegosaurus costume (yup, I'm one of those people), I found he stood a stately 12 inches tall from paw to occiput and had a perfectly proportioned head. Yet, when I try to get him into aforementioned costume, he somehow made his once normal noggin grow to the size of a watermelon.

His most impressive canine-plasticity occurs at night. When I lift him onto the bed, he is a compact little fella who just curls up and occupies a small corner of the bed. But as the night progresses, he somehow expands. By the next morning, he has grown to the size of a manatee and occupies 65 percent of prime real estate in the center of the bed, while I find myself clinging tenuously to the very edge.

So what makes a pooch's physique so malleable? Is it Hound-ini magic? Dogged determination? Or maybe it's simply body dysmorphia: The Great Pyrenees believes he's a Papillon, so he climbs into your 4-year-old daughter's lap anyway.

"I shrink, therefore I am."

Tammy Swift can be reached at tswift@forumcomm.com.