That might be because 7 is "arithmetically unique".

Flickr: pagedooley / Creative Commons

It's the only number below 10 you can't multiply or divide and keep within group. For example, 5 you can multiply by 2 to get 10 (still within the 1-10 group), 6 and 8 you can divide by 2.

17. 7 also shows up a lot in human culture.

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We have seven deadly sins, and seven wonders of the world. Not to mention colours of the rainbow, pillars of wisdom, seas, dwarves, days in the week...

This might be because when these things came about there were celestial bodies visible in the sky (the Sun, the Moon, Venus, Mercury, Mars, Jupiter and Saturn).

18. The number 4 is considered unlucky in much of Asia.

Flickr: boklm / Creative Commons

That's because the words for "four" in Japanese, Cantonese, Mandarin and Korean (shi, sei, si, sa) sound the same as the words in those languages for death.

19. .999999... = 1

Flickr: andrec / Creative Commons

Here's the proof:

If 10N = 9.9999...

Then N = .9999....

Subtract N from 10N, leaving you with 9N=9.

So then N=1. But we already know that N=.9999... as well.

So 1=.9999....

20. Cicadas use prime numbers as an evolutionary strategy.

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Flickr: oakleyoriginals / Creative Commons

Cicadas incubate underground for long periods of time – 13 or 17 years – before coming out to mate. 13 and 17 are both prime numbers. It's thought cicadas ended up in these prime number life-cycles because it meant they came into contact with predators on more round numbered life-cycles less often.