Devised by Felix Klein in 1882, it is a closed surface with no edge at all, whose inside and outside are one and the same. An ant could walk from the “inside” to the “outside” without ever crossing an edge. In our three-dimensional space, it can't be built without the neck passing through the wall. The glass models sold to the curious therefore cheat a little.
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.