As you walk along a path in a forest, the path splits in two. You do not know which way to go. At the junction, two solemn guards stand, ready to help any passersby.
It so happens that the guards only know the words yes and no. It also happens to be that the guards are twin brothers, impossible to tell apart. And, as twin brothers, the guards have developed a quite peculiar dynamic: One of them always tells the truth, while the other always lies.
You have one question, which you can ask only one of the guards. How do you determine how to continue on your path?