In 1950, 21-year-old John Nash published a paper barely two pages long defining what's now called the “Nash equilibrium”: a situation where no player gains by changing strategy alone. He proved every finite game has at least one, provided players may choose at random. The idea transformed all of economics and earned him the Nobel Prize in economics in 1994.
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.