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Two adventurers, Merlin and Hermes, approached a large iron door built into a cliff face.”Well…”, said Hermes, “What do we do now?”. Merlin produced an old, large piece of crumpled paper from his pocket. “Hrm…”, Merlin mumbled. “It says here that we must speak the six letter keyword to open the door and enter the secret chamber, but I don’t remember seeing any signs as to what that keyword might be…”

After a bit of searching, Hermes notices something etched into the ground. “Come over here!”, he yelled, pointing frantically. And sure enough, barely visible and obscured by dust, was a series of lines of different colors etched into the ground:

“Ah”, Merlin said, “So that is the keyword.” Hermes was lost and confused. After staring at it for another thirty seconds, he grumbled “What keyword!? All I see is a bunch of lines!”. Merlin simply responded, “You’re just looking at it the wrong way. It’s obvious!”

Isn’t it?

Source: Puzzling StackExchange

The signs are engraved letters on the ground and Merlin and Hermes are looking at them from above (the italic “looking at it the wrong way” is a hint). The darker a part from some sign is, the farther from the ground it is. The only letters which could correspond to this description are U – N – L – I – N – K. Therefore the keyword is “UNLINK”.

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Perplexus Original is the very first Perplexus maze created and probably the most popular one. It is rated with medium difficulty and this seems fair. Even though Perplexus Original supposedly has 100 barriers, most of them are just simple curves and drops, requiring no effort to pass. Ultimately, there are no more than 15 places along the entire track which cause difficulties, but they are still enough to pose a good challenge. In case you don’t want to start every time from the beginning of the maze – there are 2 alternative starting points in it. This allows you to place the ball at the 1st, 26th or 59th barrier, skipping all of the previous ones. The design is very good, using bright colors, making it easy to follow the track. There are several barriers which are lots of fun to pass through, such as the big spiral in the middle and few moving obstacles around. The overall construction is solid as well.

Even though Perplexus can not be called a “puzzle” in the real sense of the word, it still requires good hand-eye coordination, movement precision and intense focus. It is a great toy to have at home, especially if you have children around.

  • medium difficulty
  • 100 barriers

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A battleship starts moving at 12 PM from an integer point on the real line with constant speed, landing on every hour again on an integer point. Every day at midnight you can shoot at an arbitrary point on the real plane, trying to destroy the battleship. Can you find a strategy with which you will eventually succeed to do this?

If we know the starting point of the battleship and its speed, then we can determine its position at any time after 12 PM.

There are countably many combinations (X, Y) of starting point and speed. We can order them in the following way:

(0, 0) – starting point 0, speed 0;
(0, 1) – starting point 0, speed +1;
(1, 0) – starting point 1, speed 0;
(0, -1) – starting point 0, speed -1;
(1, 1) – starting point 1, speed +1;
(-1, 0) – starting point -1, speed 0;
(0, 2) – starting point 0, speed +2;
(1, -1) – starting point 1, speed -1;
(-1, 1) – starting point -1, speed 1;
(2, 0)- starting point 2, speed 0,
and so on. Of course, we can choose the ordering in many different ways.

Now we can start exhausting all possibilities one after another. First we assume the combination is (0, 0), calculate where the battleship would be at midnight during the first day and shoot there. Then we assume the combination is (0, 1), calculate where the battleship would be at midnight during the second day and shoot there. If we continue like this, eventually we will hit the battleship.

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I don’t know how many of you have played with and remember those wonderful cubic jigsaws from our childhoods, but they were definitely some of my favorites at that time. Escher Mirrorkal by Recent Toys looks strikingly similar to them, but a smart addition of mirrors transforms such a basic toy into an interesting puzzle which may frustrate even adults. Instead of each little cube having 6 solid painted sides, two of the sides are replaced with transparent covers and a small mirror is placed diagonally underneath them. This allows you when looking through one of the transparent sides to see a reflection of whatever is visible from the other side. Using this gimmick, your task is to arrange all 9 little cubes into the given 3 by 3 frame, transparent sides up, so that you can see one of the 5 given Escher paintings inside. The overall difficulty of the puzzle is low/medium and it gets even easier after the first image you solve. However, the wonderful concept makes the puzzle engaging and can bring you back to it every once in a while. My only problem with the toy is the fact that on my copy the stickers on the non-transparent sides are not well positioned, which makes the final pictures look a bit odd. I don’t know if this is a consistent issue or just I was unlucky, but for such relatively expensive toy, I would expect higher quality control. Nevertheless, I am more than happy with my Escher Mirrorkal and would recommend it to anyone looking for a fun little puzzle with ingenious concept and cool design.

  • medium difficulty
  • mirrors, arranging
  • fun and novel concept

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Almost everyone knows how a soccer ball looks like – there are several black regular pentagons on it and around each of them – five white regular hexagons. Around each hexagon there are three pentagons and three more hexagons. However, can you figure out how many pentagons and hexagons are there in total on the surface of the ball?

Let the number of pentagons is equal to P and the number of hexagons is equal to H. Then the number of edges is equal to (5P + 6H)/2 – that’s because every pentagon has five edges, every hexagon has 6 edges and every edge belongs to 2 sides. Also, the number of vertices is equal to (5P + 6H)/3 – that’s because every pentagon has five vertices, every hexagon has 6 vertices and every vertex belongs to 3 sides. Now using Euler’s Theorem we get P + H + (5P + 6H)/3 – (5P + 6H)/2 = 2, or equivalently P/6=2 and therefore P = 12. Since around every pentagon there are exactly 5 hexagons and around every hexagon there are exactly 3 pentagons, we get H = 5P/3 = 20. Therefore there are 12 pentagons and 20 hexagons on a soccer ball.