(1) 
Two rooms. One with 3 bulbs. One with 3 switches.
Each switch controls a bulb and each bulb is controled by a switch. Your
goal is to figure out which controls which.
At the beginning, You are in the switch room and all switches are off.
You could only go to the bulb room once but you could do any action in
each room provided that you are not going to damage something.
Could it be done?
solution



(2) 
Two glasses of same amount of water. One is blue
and the other is red.
A scientist uses a spoon to bring out a spoon of blue water, puts into
the red one and stirs it. He then uses the same spoon to bring out a
spoon of red water, puts into the blue one and stirs it.
Now he asks you which one contains more foreign substance?
solution



(3) 
Two same ropes which are used to count time. The
way the prehistoric boy used the rope was to let it be burning from one
side to the other side. The burning could last 60 minutes for each rope.
Now the prehistoric boy want to make a 45minute counter. The hard part
is that materials along the ropes are not evenly distributed, which means
that cut one rope in half may result in one 40minute counter and one
20minute counter.
So could you help him?
solution



(4) 
You have a balance and 9 sample balls. 9 sample
balls are all of the same weight except a bad sample. Unfortunately, we
don't know whether that bad one is heavier or lighter.
Could you help the sampler to pick out the bad ball in 3 balancing?
Well, if you have 12 balls instead of 9 balls could you still pick out
the bad one in 3 balancing?
solution



(5) 
Jack can't swim. But he wants to get on an island
in a lake. There is a big tree on that island. And there is another one
on the bank of the lake. Now Jack is at the bank of the lake and has nothing
but a very long rope at hand. How could he manage to get on the island?
solution



(6) 
There are 4 piles of chess pieces on the table.
Tom and Jerry, in turn, will take any amount of pieces from a pile at
a time. Whoever takes the last piece wins the game. If Jerry is the
first to act, can Tom win the game? Why?
solution



(7) 
Two identical coins are on the table side by side
as shown. Now rotate the right coin along the edge of the left coin to the
left as shown by the arrows and dotted coin. Be sure that no relative
movement happens between the edges of the two coins. Then what is the
new direction of the word "coin" after rotation? And what will happen
if we rotate the coin back to its original place through the route
underneath?
solution



(8) 
Six rooms are shown here. Those thick blue lines
are the walls. A clever robot "A" and a smart boy "B" are playing
"hideandsearch" game in one of these room. "A" is always the searcher
and rule is that "A" wins at the time "A" sees "B". Now suppose "B" is
cheating. He always knows A's future steps and will thus move accordingly.
Then for each of these rooms, will "A" win with 100% certainty? How?
solution



(9) 
Cut the left shape into two parts and piece them
together to be a 3x5 rectangle shown on the right?
solution



(10) 
Friends will to our house and play puzzles with us.
The number of friends that will come is either N or M which are relatively prime.
If we want to cut a cheese cake evenly for them beforehand, how many cuts do we
need if the cake is squareshaped? What if the cake is circular? What if the number
of friends that will come is either N or M or K which are relatively prime? Give
the smallest number of cuts possible.
solution



(11) 

(12) 
Could you find more than 10 faces in this picture?
solution



(13) 
Can you find seven horses in the picture?
solution


How about this one? (thanks to Ana from Portugal)



(14) 
Focus on the dot in the center and move your head
forward and backward. What do you see? Can you explain it?
solution


