1) Think of a number, any number. Write that number down: ________ (EG: 4261)
2) Multiply the number by 9. Write that number down: _______ (EG: 4261 x 9 = 38349)
3) Add that number's digits across Write the result down: ______ (EG: 3+8+3+4+9 = 27)
4) Repeat step 3, until all the digits added across are less than 10. Write the result down: ____ (EG: 2+7 = 9)
You answer is always 9.
You can do a lot of mind games with this trick.
But you can use this trick to figure out REALLY quick, if a number is divisible by 9.
EG: For example
     123,456 = (1+2+3+4+5+6) is 21 = (1+2) is 3, which is not divisible by 9. 123456 / 9 = 13717.333333333334
     1,234,566 = (1+2+3+4+5+6+6) is 27 = (2+7) is 9, which is divisible by 9. 1234566 / 9 = 137174
Boring, I know, but it can be useful at times.
Every number seems to have its tricks.