If you drop a buttered piece of bread, it will fall on the floor butter-side down. If a cat is dropped from a window or other high and towering place, it will land on its feet.
But what if you attach a buttered piece of bread, butter-side up to a cat’s back and toss them both out the window? Will the cat land on its feet? Or will the butter splat on the ground?
Even if you are too lazy to do the experiment yourself you should be able to deduce the obvious result. The laws of butterology demand that the butter must hit the ground, and the equally strict laws of feline aerodynamics demand that the cat can not smash its furry back. If the combined construct were to land, nature would have no way to resolve this paradox. Therefore it simply does not fall.
Thus has man discovered the secret of antigravity! A buttered cat will, when released, quickly move to a height where the forces of cat-twisting and butter repulsion are in equilibrium. This equilibrium point can be modified by scraping off some of the butter, providing lift, or removing some of the cat’s limbs, allowing descent.
Evidence suggests that most of the civilized species of the Universe already use this principle to drive their ships while within a planetary system. The loud humming heard by most sighters of UFOs is, in fact, the purring of several hundred tabbies.
The one obvious danger is, of course, if the cats manage to eat the bread off their backs they will instantly plummet. Of course the cats will land on their feet, but this usually doesn’t do them much good, since right after they make their graceful landing several tons of red-hot starship and extremely pissed-off aliens crash on top of them. No resolution to this problem is yet known.