The title sums it all.   P.S. And, as a Christmas bonus, here is a nice chess puzzle. In the above diagram you must add the two missing kings in such a way that White, who is on the move, can deliver immediate mate, i.e. mate in one move. [source...

I think that the problem stated in one of my earlier posts is one of the most fascinating puzzles I came across recently. Many people that got confronted with it said bluntly that the problem simply has no solution, otherwise it would contradict common...

There is a huge amount of aliens living on the Xen planet who want to elect their new leader (since their previous leader died a while back). They want to switch to a very democratic voting process, through the help of a very special communication field...

I love probability puzzles. Here is one I liked: There are a 100 people trying to get onto the same flight you are. The airplane has a 100 seats. You are all ready to board. You are the last one in the line of passengers at the gate. The first guy walks...

Show that (N^4 + 4^N) is a prime number if and only if N=1.

If you have an urn who already contains either a black or white ball, and you add a white ball to it, and then you subtract a white ball from it, then what is the probability of having a white ball left? [update: adding the fact that the original ball...

Here is an interesting probability problem who recently generated long discussions in our team: Say that you have an array of N boolean values, with all values initially set to FALSE. At each iteration step, you arbitrary pick an element in the array...

What is the minimum number that cannot be expressed with less than two english words? Also, how about less than thirteen english words?

Several answers to my previous puzzle reminded me about an old result: Theorem: All horses have the same color. Proof : We demonstrate this by induction over N for all the sets of horses size of size N: - N=1: The proof is true for N = 1 (any...

What is the value of this expression? If that was too easy, how about this one?

The geometry problem from my previous math puzzle has a nice solution. I particularly like it because it is one of these problems that are pretty hard to solve traditionally - unless you perform the right geometric construction - and at that point, the...

This is not really a puzzle, but a real geometry problem. Let's take a random triangle (ABC), and let's assume that the angle bisector from A intersects BC in the point D. Proof that: AD ^ 2 = AB * AC - BD * CD Here is the figure, drawn in MSPAINT...

During war, two spies (Jack and Arnold) are being parachuted in a hostile country. But during their landing, their plane gets intercepted by the special forces. Things started to go out of control: they both lose all their equipment, and Ann, the person...

You are given two white cubes. You are asked to draw a digit (from 0..9) on every square of these cubes, such that you will be able to represent evert day in a month using these cubes in certain position. For example, "13" would require that a face on...

The previous puzzle was solved correctly in the comments. So I thought to present with another prison-style puzzle: A warden meets with 23 new prisoners when they arrive. He tells them, "You may meet today and plan a strategy. But after today, you...

This is a very nice puzzle that kept me awake for a long time... :-) John and Dianne, two journalists (and experienced bridge players), are captured by some foreign government. The guardian says that he can free them, if they win the following game...

This problem initially had two parts but now has three. OK, let's start with this: let's assume that we have two, trains, 100 miles apart, each going with 20 mph to the other. A fly travels between them at 50 mph, zig-zagging just before getting smashed...

Here is a very nice puzzle - But unfortunately you have to ressurect a little chemistry knowledge to fully appreciate it... You have two identical test tubes. Both tubes look identical - they both contain colourless, odoreless, aqueous solutions. But...

Here is a fun one: The triangle ABC is isoseceles, with angle (ABC) = angle (ACB) = 80°. We draw now two segments inside the triangle. First, we choose the point D on segment AC such that angle (CBD) is 60°. Also, we choose the point E on AB, where angle...

It is known that in a certain city there are a significant number of witches, even though this practice is forbidden. The problem is that nobody knows who these witches are, since a witch is very careful to appear to others as a perfectly normal person...

You have the following false "equation", made with six toothpicks arranged on a table in the following way: XI = I You have to re-establish the equality by changing the position of a single toothpick. Common-sense rules apply: you are not allowed to bend...

You have 30 seconds left... just kidding :-) using System; using System.Collections; using System.Reflection; class Hello { Type Unknown { get { return World.GetType(); } } object [] Knowledge() { return Unknown.GetProperties(); } string [] Library()...

If you like CMD programming, here is a challenge for you: write a CMD batch file that solves the Hanoi problem. You start with these folders: and you must end up with this state: You probably know the rules of the Hanoi problem. Mapped in the command...

Mr Blobby posted an excellent comment in my previous post, outlining in a few words Gödel's proof of incompleteness of arithmetics. This is a mind-boggling result, which essentially states the following: There are certain statements (I would not say ...

Imagine that there might be a real number that contains (in an encoded form) the entire works of Shakespeare. Let's call this number a "Shakespeare number". Can you give an example of an algorithm to generate Shakespeare numbers? [update] The problem...