Decibels follow a logarithmic scale: each extra 10 dB multiplies the sound energy by 10. So two identical vacuum cleaners don't go from 70 to 140 dB, but only to about 73 dB. Conversely, 100 dB carries a thousand times more energy than 70 dB, even though the numbers look close. That's why a few decibels matter so much to your ears.
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.