A man walks down a road and comes to a fork in the road. He needs to get to the city but isn't sure which road he should take. There is a house near the junction where identical twins live. One twin always lies and the other always tells the truth. He has ONE question. What should he ask to guarantee he takes the road to the city?
This is an old chestnut - He asks one of them "If I ask the other twin which way to go, what will he say ?". He then takes the opposite direction to the one given. Work it out for yourself!