In North America, some cicadas spend 13 or 17 years underground before emerging all at once. Both are prime numbers, which, according to a popular hypothesis, limits encounters with predators on shorter cycles. As a result, a 13-year brood and a 17-year brood coincide only every 221 years. It happened in 2024 in Illinois, for the first time since 1803.
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.