Two workmen were repairing a roof. Suddenly, they both fell down the chimney and found themselves in the large fireplace.
One man`s face was smeared with soot and one wasn`t. The one with the clean face washed his and the dirty man did not and went back to work. Why?
When the two men looked at each other, the clean man thought his face was dirty as well. The dirty man, looking at the first's place, thought his was clean.
You are walking down a path when you come to two doors. Opening one of the doors will lead you to a life of prosperity and happiness, while opening the other door will lead to a life of misery and sorrow. You don't know which door leads to which life.
In front of the doors are two twin brothers who know which door leads where. One of the brothers always lies, and the other always tells the truth. You don't know which brother is the liar and which is the truth-teller.
You are allowed to ask one single question to one of the brothers (not both) to figure out which door to open.
What question should you ask?
Ask "If I asked your brother what the good door is, what would he say?"
If you ask the truth-telling brother, he will point to the bad door, because this is what the lying brother would point to.
Alternatively, if you ask the lying brother, he will also point to the bad door, because this is NOT what the truth-telling brother would point to.
So whichever door is pointed to, you should go through the other one.