Consider “the smallest number that cannot be described in fewer than fifteen words”. With a finite vocabulary, such a number must exist. But the phrase in quotes has only twelve words, so it has just described it in fewer than fifteen! Reported in 1906 by Bertrand Russell, who credited the librarian G. G. Berry, this paradox inspired deep results in computer science.
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.