Imagine you are on your way to San Francisco. Suddenly the road splits into two with one leading to the right and one to the left. At the fork there is a house where two brothers live – Mike and Frank one of whom always lies and the other one always tells the truth.
Both of them know which of the two roads lead to San Francisco but you don’t. You can ask only one single question and you can ask only one of both brothers. What question do you ask in order to definitely find out which of the two roads leads to San Francisco?