Rhubarb commented:

Have you ever noticed that as the cat settles down on your lap, knitting her claws and purring, she finally relaxes into a comfortable position and then goes deeply into sleep. While she is sleeping, she seems to get heavier and heavier, until you are pinned under a hundredweight of cat.

When, finally, you have to move, because nature calls or your leg has gone to sleep or the phone rings, it is nearly impossible to dislodge the cat. Enduring a barrage of extremely disgruntled meows and disentangling of the claws and heaving over the dead weight, you finally break free--and realize how cold the house is. Cats are wonderful heat generators.

I found myself embroidering her thoughts:

In case you haven't noticed, cats live in their own world. Not only can't you teach them to sit up, they refuse to obey the laws of physics as well. Ed Cagle, whose fanzine I read long ago, once remarked that he believed in the "Steady State" explanation for the expansion of the universe, except that new matter was not, as the scientists postulated, created inside supernovae but inside cats. He had observed that a cat would eat a pound of food and leave five pounds in the litter box. (Although he expressed himself more pungently)

As far as the conservation of energy goes (and it only goes so far) cats have their own rules. A cat at rest tends to stay at rest. A cat in motion tends to head toward a place where it can stay at rest.

When I am in the same room as our cat that place is my lap. Or, rather, the cat perch I obliging create by crossing my legs, since I have no lap otherwise.

Once it is sitting on me, the cat goes to sleep. A sleeping cat gets heavier. This is because the cat is converting every last bit of energy in its body into matter. The cat's weighty head lolls over the side of my leg. If I'm lucky it doesn't drool. (I am seldom lucky) The cat continues to droop, almost like it's melting. (Or as if it's a mass of fur covered Silly Putty) The cat's sides move slower and slower. Finally, inevitably, the increasing pressure on the nerves makes my leg go to sleep. My foot begins to tingle uncomfortably.

Petting the cat gets no response. Joggling my leg is no more effective. I try to move my leg to push the cat off and it seems like it has not only melted but stuck. I manage to get my hands under the flaccid mass of feline, grab hold and lift. Although still apparently asleep, its claws seem to have gripped my trousers.

I pull harder. The cat comes loose with a sound like Velcro. I sit it on the floor. It wobbles and slumps and blinks like it doesn't know where it is.

I manage to get up, shake my numb leg, and hobble toward the stairs. Just as I reach the top of the stairs the cat suddenly materializes under my feet and I have to grab the railing to avoid falling. I guess it's just the cat's way of reassuring me it is still alive and perhaps should be given a tin of food so that it can start creating matter for the universe again.