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Last Word is Â鶹´«Ã½â€™s long-running series in which readers give scientific answers to each other’s questions, ranging from the minutiae of everyday life to absurd astronomical hypotheticals. To answer a question or ask a new one, email lastword@newscientist.com

If all the water on Earth’s surface and in the air were put into a ball, how large would the sphere be? Could it exist in space by itself?

Jim Wegry,
Lansing, Michigan, US

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If a fit human being and a cheetah were to run a race, how long would it have to be for the human to win?

Goronwy Humphreys
Wallasey, Merseyside, UK

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To answer this question – or ask a new one – email lastword@newscientist.com.

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