True. It seems there are different versions of the puzzle, but from a quick search it was popularized by the movie Labyrinth, and there they get around it by having a second set of guards who don’t know the answer explain the setup.

Glad to help!

Yes, but you’re not asking him what his brother likes, you’re asking him what he would say he likes, which is what flips it. You’re basically making sure the answer is a lie regardless of which brother you ask.

It works the same as the original puzzle. If you ask the lying small butt brother, he’ll lie and say his brother would say he likes small butts. If you ask the truthful big butt brother, he’d say his brother would say he likes big butts, because he knows his brother likes small butts and would lie about it.

Essentially the negatives work out so that each brother answers with the kind of butt they themselves like, which you can then use to determine which is truthful (though at this point that somehow seems less important).

“If I were to ask your brother what kind of butts he likes, how would he answer?”

He asked, “Be you angels?”

We said, “Nay, we are but men!”

ROCK!

For the uninitiated