The Greeks sought to construct, using only a straightedge and compass, a square with the same area as a given circle. Generations wore themselves out on it, so much that in 1775 the Paris Academy of Sciences decided to stop examining any proposed solutions. In 1882, Ferdinand von Lindemann settled the matter: pi is a “transcendental” number, which makes the construction impossible.
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.