The reason that multiples of 9 sum to 9 is because we use decimal, or base 10, as our number system, and 9 is one less than 10. In hexadecimal, or base 16, the same effect can be seen with numbers divisible by 15 (0xf). If you were operating in base 7, all numbers divisible by 6 would sum to 6. In binary, or base 2, all numbers divisible by 1 (the integers) sum to 1. (I could explain why this is so, if anyone is interested.)
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Scissors.