Connect three houses to three utilities, water, gas and electricity, without any pipe crossing another. On a sheet of paper, it can't be done: in 1930, Kazimierz Kuratowski even proved this layout is one of the two core patterns behind every network that can't be drawn flat. But on a surface with a hole, like a mug with its handle, the problem is easily solved.
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.