Brainteasers

I like brainteasers that require creative problem solving. This list does not include

• Logic puzzles that require mechanical evaluation because they are just tedious.
• Lateral thinking problems because they rarely have a truly unique answer.
• Riddles that exploit deficiencies in human psychology because they are not really educational.

1. You have an 8x8 chess board and a pile of dominos. Each domino covers exactly two squares on the chess board. Prove whether it is or is not possible to cover every square of the chess board with dominos except the top left and bottom right squares.
2. You are given a cylindrical can with no top and you are asked to fill it exactly halfway full with water from a faucet. You have no other objects and there is no way to make markings on the can.
3. Before you are a glass of wine and a glass of water with the same amount of liquid in each. You take a spoonful of wine from the wine glass and dump it into the water glass and then take a spoonful from the water glass and dump it into the wine glass. What is the ratio of wine in the water glass to water in the wine glass?
4. You have ten pennies, but one is a fake and has a different weight. How can you find the fake using only three weighings on a balance?
5. You would like to cut a cake into two halves of equal volume, but someone has already cut a piece out of the middle of it. Without moving the cake, how can you do it?
6. There are two doorways in front of you. The door on the left leads to a room with three lightswitches labelled A, B, C. The door on the right leads to a room with three lightbulbs labelled X, Y, Z. The swithes connect to the lightbulbs, but you don't know which switch connects to each lightbulb. Initially, all the switches are down and all the lightbulbs are off. Once you leave the switch room you cannot re-enter it. How can figure out the connections between A, B, C and X, Y, Z?
7. You have a table with five phonebooks laying on it. They are 6, 5, 4, 3, 1 cm thick respectively. How can you create a pile 17 cm high?
8. Connect a square grid of nine dots with four straight lines.
9. A bum collects cigarette butts from the street and extracts the tobacco to roll new cigarettes. For every 7 butts he finds, he can roll a new cigarette. How many smokes can he get out of 49 butts?
10. You have two hourglass timers. One takes seven minutes for the sand to run through and the other takes 11 minutes. How do you time for exactly 15 minutes?
11. Coming Soon: Three people with 5 black and white discs.
12. Coming Soon: Brothers at crossroads, one always lies and one always tells the truth.