How many times must you riffle-shuffle a 52-card deck, splitting it in two and interleaving the halves, before it is truly mixed? In 1992, Dave Bayer and Persi Diaconis showed the answer is seven. With fewer, traces of the original order survive and can be exploited by an observant player. Beyond seven, each extra shuffle adds almost nothing.
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.