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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

Binocular vision gives us a 3D perspective, and two ears let us locate the direction of a sound. But why two nostrils?

Richard Kubiak,

Usk, Monmouthshire, UK

How long would it take to accelerate a spacecraft to 99 per cent light speed without major injury to the astronauts inside?

John McKain,

Glasgow, UK

To answer this question –…