The world's hardest games and puzzles: from GCHQ to Einstein's riddle

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From fiendishly difficult riddles to a Sudoku unsolvable to all but the sharpest of minds, we've rounded up some of the world's toughest conundrums to celebrate National Puzzle Day.

Unsure how to approach some of the more infuriatingly complex brainteasers? Sir David Omand, the former director of the British intelligence and security agency GCHQ, shares some tips:

"When it comes to cracking codes, what is so useful is a capacity for sustained thinking – holding a problem in one's mind, examining it from every angle," he writes in the Daily Telegraph.

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"You have to let past experience and inspiration bubble up to suggest possible ways in and apply rigorous intellectual honesty to the result."

Failing that, there are some clues and solutions here...