Many people refuse to believe it, yet it isn't an approximation. One third is 0.333… with endless threes; three thirds make 1, and also 0.999… Another argument: if two numbers are different, you can always squeeze a third one between them. But between 0.999… and 1 there is nothing at all: they're two ways of writing the same number.
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.