This question was posted to the Usenet Oracle:

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 cats back and toss them both out the window? Will the cat
land on its feet? Or will the butter splat on the ground?

And in response, thus spake the Oracle:

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.

Thats right you clever mortal (well, as clever as a mortal can get), you have
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 cats limbs, allowing
descent.

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 doesnt do them much good, since right after they make their
graceful landing several tons of red-hot starship and pissed off aliens crash on
top of them.