It's about time I wrote this down. Since I'd just formulated this in a reply in mizkit's journal, I thought I'd repeat it here:
sounding like a million cats in army boots.
I have this theory. OK, perhaps only a hypothesis, since nothing is falsifiable when cats are around.
It's called the Feline Conservation of Weight Principle, and it goes like this:
Cats are averagely proportioned small animals, with a weight according to the physical principles of size, density and local gravitational field. However, they are exceptional hunters, extremely light on their feet when stalking, and so on.
It is obvious to an observer that a cat weighs less when stalking. We know this is not due to the gravitational field changing, since the prey don't suddenly also develop strange lightness, so it must be that the cat now has less mass. This is obviously contrary to the normal laws of physics, particularly the law of conservation of mass. Except that the conservation laws are actually slightly bendable - for instance, a subatomic particle may come into existence from nowhere for a suitably brief period, just so long as it disappears again - and this is one of those cases. So long as the average weight of the cat is constant over the longer term, it may vary up and down. When stalking, some of its weight is 'postponed' to the future.
The 'cats in army boots' phenomenon is merely the time that the extra weight comes into effect to rebalance the average.