Notice the relationship between the product of the 9's tables from to 10 and the sum of the digits of each product.  The sums of the digits of each product equals 9.  Does anyone know why this is the case?

1 X 9 = 9    0 + 9 = 9
2 X 9 = 18  1 + 8 = 9
3 X 9 = 27   2 + 7 = 9
4 X 9 = 36   3 + 6 = 9
5 X 9 = 45   4 + 5 = 9
6 X 9 = 54   5 + 4 = 9
7 X 9 = 63   6 + 3 = 9
8 X 9 = 72   7 + 2 = 9
9 X 9 = 81    8 + 1 = 9
10 X 9 = 90  9 + 0 = 9