Question: There are 4 light bulbs in a room and 4 switches corresponding to these four light bulbs (in a random order). You cannot see the light bulbs from where the switches are.
If you can only visit the room with the light bulbs once, how can you figure out which switch belongs to which light bulb?
Answer: Switch on two of the light bulbs for a long enough period of time for them to get hot. Then switch off one of those light bulbs and switch on one of the light bulbs that was off. At this point there will be a light bulb that is hot and on, hot and off, cold and on, and cold and off (allowing you to figure out which bulb goes to which switch).
Question: There were 2 doors. Behind the 1 door, is hell and behind the other door, is heaven but you don't know which door will take you to heaven. In front of them, there were 2 brothers, which is guarding the door. One of the brother always lie and the other one always tell the truth. Of course you don't know who is lying and who is not. You only get 1 question to ask one of them to figure out which door leads to heaven. What question you might ask? Remember you only get to ask 1 question. it means that you can't ask one question each of them. that will be 2 questions. You only ask 1 question to any of them and no more. How do you do that?
Answer: Go up to one of the guy and ask this question, " Hello, which door that your brother will point if I ask him which way is the heaven." Then take the other door. Don't enter the door that he was pointing. This question will work for both of them because if you ask this question to the truth guy, he will point the hell because he knows that his brother will point hell and if you asked to the liar, he'll still point hell because he knows that his brother will point to heaven door, so he lied. That's why you take the other door.