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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

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In winter, birds often sit quietly on a branch for hours. Do they actually sleep more or is it just a way to conserve energy?

Caroline Deforche
Lichtervelde, Belgium

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Why do we scream in fear? It doesn’t help and it makes us more vulnerable to predators.

Chadi Ajaj
ÌýOttawa, Canada

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

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