Maths has a ton of fascinating quirks. For example, if you want to convert between miles and kilometres, you can use the Fibonacci sequence to make a conversion with a stunning degree of accuracy.
Photo by Chad Elliott.
0, 1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89, 144
Each number in the sequence after the first two is the sum of the previous two numbers. Coincidentally, it works as a nearly-accurate converter between miles and kilometres. Look at any number in the sequence as a number of miles and the next number in the sequence is approximately that same distance in kilometres. For example, 8km is about 13km. Thirteen miles is about the same as 21km.
As Reddit user daneah explains, this is because the golden ratio (~1.618) is very close to the conversion of miles to kilometres (~1.609). Of course, that tiny difference will break down eventually, but for everyday mental conversions, it's handy.