<< heres a brainteaser,
Two boxes are labelled "A" and "B". A sign on box A says "The sign on box B is true and the gold is in box A". A sign on box B says "The sign on box A is false and the gold is in box A". Assuming there is gold in one of the boxes, which box contains the gold? >>
No, I've got it.
I ask Alph what Ralph would say if I ask which door leads to the castle.
If Alph is telling the truth when he says that Ralph lies, then it must be the opposite of the door he answers with.
If Alph is lying when he says Ralph lies, then Ralph is telling the truth, but Alph lies, so it STILL must be the opposite of the door he answers with.
I've got it.
This is the way to the castle of the goblin king!