The digital root of a number is found by adding its digits repeatedly until you get a single digit.
When you do this with multiples of 9, the result always comes back to 9.
1+8 = 9
2+7 = 9
3+6 = 9
4+5 = 9
9+9=18 => 1+8 = 9
Cats use this to get extra sets of 9 lives
