You are lost in the woods and the path you are following forks into two: a path to the left and one to the right. One path will lead you to safety while the other will cause you to be lost forever.
At the fork are two twin sisters who know which path is which. The sisters are identical in every way except one: one of the sisters always tells the truth while the other always lies.
You can ask only one question and you don’t know which sister is which. What can you say to them so you know which path to take?
Ask one of the sisters a question...
Ask one of the sisters which path their sister would tell you to take.
Let’s say the left hand path is the correct one to follow. The sister that lies knows their truthful sister would tell you the left hand path, so as they always lie they will tell you the right hand path.
The honest sister knows their lying sister will tell you the right hand path and because they’re honest, they will tell you this. So you should follow the opposite path to that which you are told, regardless of which sister tells you.