So to yesterday's problem:
"You come to the gates of heaven, and there are two doors. St. Peter explaines that one door leads to Heaven, and one to Hell. There are twin brothers guarding the doors, one of whom never lies ("a knight") and the other never tells truth ("a knave"). You may ask one brother one question, and then must select a door. What question do you ask?
How's this one:
"What will your brother say?"
If the answer is "he'll say he's guarding Heaven", go through this brother's door, if the answer is "he'll say he's guarding Hell", go through the other. Works whichever brother you have, and whichever door...