The “hairy ball theorem”, proven by Brouwer in 1912, states that you cannot comb a hairy sphere flat without creating at least one cowlick. A surprising consequence: the wind blowing across Earth's surface must, at every moment, drop to zero somewhere. So there is always at least one point on the globe where the horizontal wind is exactly nil.
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.