A genealogist is researching two non-intersecting family trees. Members of one family always tell the truth, and members of the other family always lie. The genealogist comes to a brick wall. Standing at this wall is a member of each family, but the genealogist can't tell which family each belongs to. What question should she ask to break through the wall?
Sigh…to belong to families of all truth-tellers, or all liars. That would make research so much easier.