Happy Lunar New Year of the Rabbit! (Make sure to click on the image and zoom in to see details in the high-res card.)
In 1202, Leonardo of Pisa, known as Fibonacci, calculated the growth of an idealized (biologically unrealistic) rabbit population:
Suppose a newly-born pair of rabbits, one male, one female, are put in a field. Rabbits are able to mate at the age of one month so that at the end of its second month a female can produce another pair of rabbits. Assuming that rabbits never die and a mating pair always produces one new pair (one male, one female) every month from the second month on, the puzzle that Fibonacci posed was: how many pairs will there be in one year?
The answer, as we know today, is the 13th number of the Fibonacci sequence: 233.