The Travellers' Dilemma: a decision paradox that still puzzles game theorists.
Why does the moon appear larger when near the horizon? This illusion still baffles the boffins. Written and posted in November 2013.
Why does the number 1 occur as the first digit 6.5 times more often than 9 in data as varied as electricity bills, lengths of rivers, and rotation rates of pulsars? Benford's Law.
How to cross the bay without sexual pecadillos puzzle.
The deceptive prize swap paradox.
The formidable and famous Two Envelope Paradox is a simple but very challenging conundrum, one that the philosophers are still arguing about.
An elementary but tricky three card puzzle
The disappearing 13th man visual puzzle
Lateral thinking: a medley of short puzzles.
Cogito ergo sum?
Bertrand's Paradox in simple probability.
Newcomb's Paradox: the philosophers are still arguing about this one.
The Monty Hall Puzzle: a difficult puzzle on probability.
The Liar's Paradox: a classical Greek paradox that tormented and inspired Bertrand Russell.
Birds of diverse feathers: a difficult multiple-choice test devised by Tad Boniecki.
A ternary duel: whom would you shoot first?
Proof that 2 = 1 and other mathematical foolishness.
U2 in a hurry: can you maximise their bridge crossing capability?
The Prisoners' Dilemma: a famous decision paradox.
The St Petersburg Puzzle: an infinitely expensive gamble.
The Sleeping Beauty Problem is a difficult but elementary problem in probability.
If all the above were too easy then have a go at the Millenium Problems.