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Nine is a number that has a really cool finger trick to memorize the first numbers that have 9 as a factor! Here is how you do it. Note - it only works up to 90.

You number your fingers from one to ten. From there, put it down. You will then see the tens on the left side and ones on the second. As an example, if you put your 7th finger down, you will have six on one side and three on the other, for 63.

Anyway, there is nothing else cool about this number.