Cross-section of a fossilised ammonite shell

As easy as 1, 1, 2, 3…. Fibonacci Day

It's a bit of a fib that Fibonacci, the 13th-century Italian mathematician, was the first to create a sequence by adding each number to the preceding number: 1, 1, 2, 3, 5, 8, 13 and so on, forever. In fact, Hindu scholars described the sequence centuries before him - and they probably weren't the first to figure it out either. Regardless, 23 November, which is written 11/23 in the US style, is the day people celebrate the infinite series known as the Fibonacci sequence.

To help you picture it: If you arrange squares of measuring 1x1, 1x1, 2x2, 3x3, 5x5, 8x8, etc, on graph paper, a curved line drawn through each square will form a perfect expanding spiral not unlike the ammonite fossil cross-sectioned here. Not every spiral in nature expresses a perfect Fibonacci sequence, but nature does seem to have a thing for spirals. And in that sense, the Fibonacci sequence seems especially elegant.