Roughly three thousand years ago, a great flood happened. The people, sensing that the river god was upset, tried to offer sacrifices to calm his anger. Each time they made an offering, a turtle would appear from the river.
It had a curious figure/pattern on its shell; there were circular dots of numbers that were arranged in a three by three nine-grid pattern such that the sum of the numbers in each row, column and diagonal was the same: 15. This number was also equal to the number of days in each of the 24 cycles of tropical year. This pattern, in a certain way, was used by the people to control the river.
After studying these markings the people realized the correct amount of sacrifices to make - 15 - and consequently, the river god was placated.This pattern is called a "magic square", in that each column and the two diagonals add up to the same number.
