Ok, here's what's up.

My cats like butter.  Or margarine.   I'm not sure why this is, but they
like them both.  

But I got to thinking.  On one hand, both are fat based (be it vegetable
or animal), and strike me as something that just might also help as a sort
of hairball remedy.

On the other hand, butter is a dairy product.  Which means lactose.  Which
means I suddenly get methane propelled cats.  Pfew.

In short, jokes about perpetual motion machines aside, are small amounts
of butter a good thing or a bad thing for my cats' diet?