Right. So you're walking along the pathway to Heaven, but you come to a fork in the road. One pathway leads to Heaven, the other, to Hell. You have no way of telling which is which, but it's guarded by 2 brothers, identical in every way, other than that one always tell the truth, and the other always lies, but you don't know which. To find the correct route, you can ask on ebrother one question. What should you ask?
Answer - point to one pathway, and ask a brother, 'if I asked your brother if this is the way to Heaven, what would he say?' If the response is 'no,' then it's the correct way, and that's the pathway to choose.
Why? Because if the brother you asked was the one who always lies, he'd know it was the correct route, and so know his brother would say 'yes.' His response would be 'no,' as he'd have to lie about what his brother would say. If you'd ask the brother who tells the truth, he'd know that his brother would lie about it being the correct way, so he'd truthfully tell you that his brother would say no. I hope that made sense!!