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.
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.
The so-called Arabic numerals, which reached Europe from India via the Arab world, took a long time to catch on. In 1299, Florence's guild of money changers forbade members from using them in their account books. A commonly cited fear: a 0 could too easily be turned into a 6 or a 9. Amounts had to be written in Roman numerals or spelled out in words.