If you put a coin in an empty bottle and insert a cork into the neck of the bottle, how could you remove the coin without taking the cork out or breaking the bottle?
You want to send a valuable object to a friend securely. You have a box which can be fitted with multiple locks, and you have several locks and their corresponding keys. However, your friend does not have any keys to your locks, and if you send a key in an unlocked box, the key could be copied en route. How can you send the object securely?
You've been sentenced to death in an obscure foreign country which has a strange law. Before the sentence is carried out, two papers -- one with "LIFE" written on it and one with "DEATH" written on it -- are folded up and placed in a hat. You are permitted to pick out one of the papers (without looking), and if you choose the one with "LIFE" written on it, you are set free. Otherwise, the death sentence is carried out. On this occasion, a mean-spirited acquaintenance of yours, bent on your demise, has substituted the paper with "LIFE" written on it with another one with "DEATH" written on it. This person gleefully informs you of what he has done and that you are doomed to die. You are not permitted to speak to anyone about this misdeed, nor will you have a chance to switch the papers or the hat yourself in time. How will you avoid certain death?
You have an old-fashioned refrigerator with a small freezer compartment capable of holding seven ice cube trays stacked vertically. But there are no shelves to separate the trays, and if you stack one tray on top of another before the ice cubes in the bottom tray are fully frozen, the top tray will nestle into it, and you won't get full cubes in the bottom tray. You have an unlimited supply of trays, each of which can make a dozen cubes. What's the fastest way to make full-sized ice cubes?
Why is it better to have round manhole covers than square ones?
Four switches can be turned on or off. One is the light switch for the incandescent overhead light in the next room, which is initially off, but you don't know which. The other three switches do nothing. From the room with the switches in it, you can't see whether the light in the next room is turned on or off. You may flip the switches as often and as many times as you like, but once you enter the next room to check on the light, you must be able to say which switch controls the light without flipping the switches any further. (And you can't open the door without entering, either!) How can you determine which switch controls the light?
Three surgeons and a clumsy cook go camping in the remote wilderness. The clumsy cook stumbles over the campfire as he is serving the surgeons, injuring himself and dumping hot stew on the hands of the surgeons. The cook's injuries need surgical treatment. The surgeons' injuries are minor but open. It turns out they brought the equipment necessary for the cook's surgery with them, and they can use the campfire to sterilize the tools. But there are only two rubber gloves. Because of the different surgeons' skills, all three of the surgeons are needed to operate on the cook, in sequence. How can this be done without any of them being exposed to the blood of any of the others?
Two fifty foot ropes are suspended from a forty foot ceiling twenty feet apart. You have only a knife. How much of the rope can you steal?
Two people are talking on the phone long distance. One is in an East Coast state of the U.S., the other is in a West Coast state of the U.S. The first asks the other, "What time is it?" He hears the answer and says, "That's funny. It's the same time here!" Neither one of them were mistaken about the time. How is this possible?
A set of three light switches are located in the first floor of a building. Only one of them turns on a light on the second floor. The other two switches do nothing. If you can only go up the stairs one time, and you can't see the second floor light from the first floor, how can you be sure which switch turns on the second floor light?