“Well, I don’t know.”  Twilight shifted into a more comfortable stance.  “There’s lots of plants that can make ponies sick, and of course we know what they look like and smell like, so we don’t eat them by mistake.  We can’t eat some things that ruminants can, and they can’t eat some things that we do.  So, it makes sense—like Winter Rye suggested in one of his books—that a completely alien species could be unable to digest pony food at all.”