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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

What is it about the human physiology that leads us to use pillows, and does anyone know when we first came up with the idea?
Paul Bethel
London, UK

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Why do jewellery chains put in a box together become so tightly knotted on themselves, and each other, that it is impossible to unknot them?
Pat Critchley
London, UK

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

Questions should be scientific enquiries about everyday phenomena, and both questions and answers should be concise. We reserve the right to edit items for clarity and style. Please include a postal address, daytime telephone number and email address.

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