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(Credit: Getty Images)

(Credit: Getty Images)

(Credit: iStock)

(Credit: Getty Images)

(Credit: Nigel Hawtin)

Sources:  Nasa and other sources

Read our last SmartList, revelaing six ways to boost your willpower.

What did you learn this week? Share your most mind-expanding facts with us on Twitter with the hashtag #SmartList. They could be included in future posts…

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