Q.2 : You want to send a valuable object to a friend. You have a box which is more than large enough to contain the object. You have several locks with keys. The box has a locking ring which is more than large enough to have a lock attached. But your friend does not have the key to any lock that you have. How do you do it? Note that you cannot send a key in an unlocked box, since it might be copied.
Q.7 : You are standing in front of a room with one lightbulb inside of it. You cannot see if it is on or off. Outside the room there are 3 switches in the off positions. You may turn the switches any way you want to. You stop turning the switches, enter the room and know which switch controls the lightbulb. How?
You turn 2 switches 'on' and leave 1 switch 'off' and wait about a minute. Then enter the room, but just before you enter, turn one switch from 'on' to 'off'. Once in the room, feel the lightbulb - if it is warm, but off, it has to be the last switch you turned off. If it is on, it has to be the switch left on. If it is cold and is off, it has to be the switch you left in the off position.
Q.8 : A prisoner is told 'If you tell a lie we will hang you, if you tell the truth we will shoot you.' What can he say to save himself?
Q.9 : In a contest, four fruits (an apple, a banana, an orange, and a pear) have been placed in four closed boxes (one fruit per box). People may guess which fruit is in which box. 123 people participate in the contest. When the boxes are opened, it turns out that 43 people have guessed none of the fruits correctly, 39 people have guessed one fruit correctly, and 31 people have guessed two fruits correctly. The Question: How many people have guessed three fruits correctly, and how many people have guessed four fruits correctly?
It is not possible to guess only three fruits correctly: the fourth fruit is then correct too! So nobody has guessed three fruits correctly and 123-43-39-31 = 10 people have guessed four fruits correctly.
Q.10 : There was a man who was born before his father, killed his mother, and married his sister. Yet, there was nothing wrong with what he had done. Why?
His father was in front of him when he was born, therefore he was born before him. His mother died while giving birth to him. Finally, he grew up to be a minister and married his sister at her ceremony.