In the 17th century, Antoine and Bonaventure Rossignol created a code for Louis XIV in which hundreds of numbers stood not for letters but for syllables, with traps to mislead the enemy. After their deaths, the key was lost, and royal letters became unreadable. Only in the 1890s did cryptologist Étienne Bazeries crack it, after three years of work.
How many different structures can you build with six classic 2-by-4 Lego bricks of the same colour? The company used to quote just over 102 million. In 2005, Danish mathematician Søren Eilers recounted everything by computer: 915,103,765 combinations. The official estimate had been too low by a factor of about nine.
Fold an A4 sheet in half and you get an A5 with the same proportions. That only works for one ratio between length and width: about 1.414, the square root of 2. The A0 format measures exactly one square metre, and A4 is a sixteenth of it. With standard 80-gram-per-square-metre paper, an A4 sheet therefore weighs exactly 5 grams.