We all know the obvious Kosher laws. Pork: not Kosher. Combining milk and meat: not Kosher. Shellfish: not Kosher. Insects (except for the delicious locust): not Kosher.
But what about the more obscure foods? For instance... bats?
Well, the ancient Hebrews were not exactly up to speed with animal qualification. They considered bats to be birds. And the basic rule is, birds of prey: not Kosher.
So, if one would bite the head of, oh, let's say a dove, that would be fine with the laws of kashrut, because doves are not birds of prey. But if one would do that to a bat (a real life bat, even if that one thinks it's rubber), that would not be Kosher at all.
Of course, not all Jews obey all Kosher laws (we love a little lobster, ourselves). So just because someone takes a bite of a bat, doesn't necessarily make them not Jewish. Just a bit... odd.
Which means that even someone culinary peculiar as Ozzy Osbourne — who is married to a Jewish woman and who recently renewed his wedding vows in a Jewish ceremony — would be welcome among us if he was Jewish. But he isn't. And not even an all-matzah diet is going to change that.