Now, I'm not much of a maths fan. But I am good at it. And I am a numbers fan. Nine is one of the cooler numbers around, really. Though, admittedly, it's only so cool because it's a base-10 number system. (For those unaware, that just means that there are ten digits - 0, 1, 2, 3, 4, 5, 6, 7, 8, 9. Binary, on the other hand, is a base-2 number system.)
Now, the big cool thing about nine is to do with something called a digit sum. A digit sum is basically taking all the digits in a number, and continually adding them together until you only have one digit left. The digit sum of any multiple of 9 is also 9. As such, it's a very handy way of making digit sums very easy, even with long numbers. You can take out the 0s straight away. Then you take out all the 9s. Then you pair up other numbers to make 9s - 4s and 5s, 3s and 6s, 2s and 7s, 1s and 8s. Whatever you've got left over is pretty simple, really. Unless you've just got lots of the same number, then it's just annoying.
For example, this number - 786,050,256,256,705,202,500,575,903,598,792,256. I just randomly typed the number keys, then added the commas in. Taking the 0s and 9s out already gets it down a lot. 78652562567522557535872256. Then the pairs. 8655656525555825. Double 5s are 1. 86662825. Triple 6 is 18, a multiple of 9. Easy takeout. And we're left with 82825. That's two tens and a 5, which is 25, which is 7. Too easy.
Nine is also the first odd non-prime number. Unless you count 1. Which you probably should. :P
It's cool, OK?
Yes, I'm a bit of a maths nerd. I've also, in my spare time, attempted to figure out a formula to calculate the number of squares of any size in a square grid, and the same thing for triangles in a triangle grid. That one's hard.