Everybody loves a good puzzle. There’s a certain satisfaction in figuring out the answer to a difficult riddle. Plus, research shows riddles and puzzles keep your brain sharp in old age.

The internet is a repository of mind-bending puzzles. But many of these require you to have technical skills, like looking up the source code of a page for clues or altering image files. Logic should test your brain, not your computer skills.

We can’t say if these are the “hardest” puzzles online. What we can say for sure is that these five brain-teasing sites are loved by anyone who likes solving stuff.

## NSA’s Puzzle Periodical (Web): One Difficult New Puzzle, Every Month

The National Security Agency has a bad rep among internet users because of its spying and infringements of privacy There are several Internet activism groups who are fighting on your behalf for privacy. They are doing their best to educate netizens as well. Here are just a few of them that are incredibly active. Read More . Set that aside for a minute and you’ll know that some of the brightest minds work there. And every month, one of these bright minds submits a puzzle for the world to figure out.

The NSA Puzzle Periodical was started only last year, so there isn’t a large collection waiting for you. But you can still browse through logic puzzles, written precisely and exquisitely. Most importantly, it’s a new puzzle every month!

The answer is posted a few days after the question, so feel free to discuss it with others till then. There are no trick questions or cheap answers, as far as I can make out.

## Sudoku Escargot (Web): The Hardest Sudoku Ever

You’re probably familiar with the numbers-oriented game of Sudoku. It’s a stress-free way to give your brain a 5-minute workout Keeping your brain in top shape requires that you challenge it on a regular basis. Surprisingly, you can do a lot in five minutes. Here are five things you can do for your mental fitness. Read More . You might already be pretty good at it. But do you think you can solve the hardest Sudoku puzzle ever made?

Mathematician Arto Inkala built an algorithm to generate challenging Sudokus, called AI Sudoku. And this is the most difficult one the bot ever created. It’s called the Escargot, and on his blog, Inkala explains why it’s the most difficult Sudoku.

Try solving it on Sudoku Wiki or print it out and take it with you. No matter how you choose to tackle this, make sure you have plenty of time. And Inkala’s blog post linked above also has 19 other super-difficult Sudokus for you to solve.

## Blue Eyes (Web): XKCD’s “Hardest Logic Puzzle”

I love getting a difficult puzzle that I can slowly work on, over several days or weeks. The joy of a good riddle lies as much in the journey. If you’re patient with puzzles, then read “the hardest logic puzzle in the world,” according to XKCD.

For the uninitiated, XKCD is one of the best webcomics for geeks Webcomics are arguably more popular now than they ever have been before. This is mainly due to a marked increase in the quality of content, with some webcomics being both magnificently written and magnificently drawn.... Read More , often talking about logic, math, and lateral thinking. Its creator shared this puzzle (which he heard from someone else) in the simplest language possible. No word play, no double meanings, or anything else.

Read the riddle, digest every information from it, and set about working it out in your head. You’ll find yourself thinking about it whenever you have some time.

## 101 Friday Puzzles (Web): Richard Wiseman’s 101 Brainteasers

Noted psychologist Richard Wiseman is a bit of an internet celebrity. He is known for amazing illusions that blow your mind The internet is full of magic. Amazing music videos, magic tricks, and optical illusions done with the help of cutting edge technology will make you go "wow". Read More on his YouTube channel, he is a noted expert on magic and psychology, and he is a famed logician.

Every Friday, Wiseman shares a puzzle or riddle on his blog, challenging readers to solve it. The puzzles use a combination of linear and lateral thinking, so some answers will require creative thinking.

You’ll find matchstick problems, logical riddles, and some picture-based puzzles too. There are 101 to get through, so save this page for offline reading wherever.

## Logic Mazes (Web): Robert Abbott’s Famous Puzzles

Logician, programmer, and game inventor Robert Abbott has been designing games since the 1950s. Abbott pioneered the concept of “logic mazes”, and some of his best ones are available online for free.

A logic maze is a maze or a grid with some set rules. For example, the Easy Maze 1 dictates you can never turn left. So without turning left, how do you travel from the start to the finish line?

Abbott also makes interactive puzzles that are hosted on the site, so you can “play” instead them to solve. Trust me, you’re going to need that little interactivity if you want any hope of getting through these.

## Can You Solve This?

You can solve any of the logic questions above, or you can try your hand at figuring out this fun puzzle.

You have 16 marbles and a balancing scale. One of the marbles is heavier or lighter than the others, while 15 are the same weight. Using the scales three times or less, identify the odd marble and whether it’s heavier or lighter.

1. Desiree c
August 19, 2017 at 10:30 am

For the maze, you could go up and then down which would make the left a right coming from the opposite direction. So if you would need to make a left just go up and come back down the same path and it'll be a right. Right??

• rahul
October 12, 2017 at 4:07 am

Why thought moving back word is the solution.

2. Desiree c
August 19, 2017 at 10:27 am

For the maze, you could go up but then go down the same way making left a right. So that every time you have to make a left you just come the opposite way making it a right. Ya know?

3. korosh
June 20, 2017 at 6:45 pm

(1)2*8 =>(2)2*3=>(3)1~1,1

4. Gabriel
June 4, 2017 at 1:29 pm

Well, take out one of the marbles at random.Now we have 15 marbles and one which is different or not.Divide the 15 marbles into 3 equal piles.
Case 1: If the one we took out was the different one,then by 2 comparisons of the 3 equal piles we can say that the one we took is the different one and comparing it with any other marble we can say how it differs (heavy or light).Thus by weighing 3 times we could solve it.
Case 2: If the different marble is in one of the piles, we can easily compare the 3 piles to find
how the whole pile containing the different marble weights lighter or heavier.Thus by weighing 2 times we could solve it:)

5. WG CDR THOMAS WALKER
April 26, 2017 at 11:50 am

Can not be done with 16 Marbles in 3 weighings !

Only 12 Marbles can be handled in 3 weighings !

• Danny
June 26, 2017 at 6:44 pm

You are right it cannot be done but the maximum number of marbles in 3 weighings is in fact 13.

• Danny
June 26, 2017 at 7:08 pm

• Danny
June 26, 2017 at 7:10 pm

• Thomas Walker
June 27, 2017 at 4:09 am

If only a simple balance is used weighing the given balls against each other then only 12 balls can be handled in 3 weighings.

If extra standard balls are available say 9 more then 13 balls can be solved in 3 weighings.

• Danny
June 26, 2017 at 7:07 pm

This puzzle is impossible.

I can only imagine there is a typo which should have read 12 marbles. Only a maximum of 12 marbles can be solved in 3 weighings to find the odd marble and tell whether it is lighter or heavier. You have to go to four weighings to solve 13 to 39 marbles.

6. Lill Venke Hustvedt
October 21, 2016 at 8:34 pm

1: 6 + 6 marbles. If the same weight, the heavy marble is one of the 4 at the table
2: 2 + 2 marbles.
3: 1 + 1 from the heaviest side, and there it is
Or
If one of the 6 are heavier:
2: 3 + 3 marbles on the scale.
3: 1 + 1 marble on the scale from the heaviest side, and one at the table. If scale is levelled; the heavy marble is at the table, if not; the scale will tell
:-)

• Robert
October 23, 2016 at 12:11 am

That only finds the heavier one, but doesn't solve the problem for a lighter one.

7. Raj
September 14, 2016 at 3:08 pm

7 7 at both sides
So here r 3 equation

1,. 7 = 7 then rest 2 weight 1 1 we will get the and in weights
2,. 7 >7
3 3. 1
If sq then 1

3 , 7 left is lighter than 7 wirigjt

• Vincent McAlpine
November 9, 2017 at 1:04 pm

Let's say left scale is heavier. Which party are you going to choose to scale further ? The heavy group or the light group ? It can be either one.

8. Anonymous
August 30, 2016 at 8:30 pm

It's always easier, at least emotionally, to solve/(re)solve logical puzzles than logical problems.

Puzzles come with an assumed advantage of knowing there is a solution with very little life consequences/conclusions involved in the solving.

9. Anonymous
August 30, 2016 at 11:29 am

Very interesting puzzles. I could play them to train my brain.

10. Raj Lath
August 30, 2016 at 7:44 am

I will have 6 marbles on both side of scales
If they are equal
I will have rest 4 with 2 in each side
whichever side is heavier
i will have one in each side from heavier lot
If any of the side having 6 marble is heavier
I will pick those 6
Will have 3 each in both the sides
From whichever is heavier i will pick two and have them 1 each in both sides.
If they are equal third one is heavier

• Robert
October 23, 2016 at 12:12 am

This doesn't work if one is lighter.

• Raj Lath
October 23, 2016 at 3:37 am

Yes i should have replaced heavier with heavier or lighter