Read

The history behind the puzzles, how they work, and ways to think about solving them. We only write what we could check, with sources.

• The story of the birthday paradox With just 23 people, the chance that two share a birthday is already over one half. Why intuition is off, and how it matters for cryptography.
• The story of the 15 puzzle Exactly half of all arrangements can never be solved. Why that was proved in 1879, and who really invented the puzzle.
• The story of the Galton board One ball is unpredictable, but many balls pile up into a fixed bell shape. How chance builds order.
• The story of Hakoiri Musume Is the shortest solution 81 moves or 116? How counting changes the answer, and one puzzle that wears a different story in every country.
• The story of the three doors Switching doubles your chance of winning. The counter-intuitive probability puzzle that set off a big argument in a 1990 magazine column.
• The story of Number Place From an American puzzle magazine in 1979, to Japan, to the world. And how a computer proved that 17 clues is the minimum.
• The story of pentominoes Twelve pieces, 2,339 ways to fill a 6×10 rectangle, and only 2 for 3×20. How the name works, and how counting rules change the answer.
• The story of river-crossing puzzles A river-crossing puzzle appears in a collection more than 1,000 years old. The trick is bringing something back, and only the cast has changed over the centuries.
• The story of the tangram Seven pieces make exactly 13 convex shapes, no more. A trade-ship journey from China to the West, and the made-up tale of a god from 4,000 years ago.
• The story of tetrominoes Four squares can be joined in just five ways. Why all five can never make a rectangle shows up when you count the black and white squares of a checkerboard.
• The story of the Tower of Hanoi Each extra disk roughly doubles the work. With 64 disks it would take about 585 billion years, says a story that came with the puzzle.
• The story of the water jug puzzle Measure an exact amount with two unmarked jugs. What divides the amounts you can make from those you cannot is the greatest common divisor.