Horsing about

When I was younger, my mum used to drive us past a field with a horse in it. There was a sign that read: "This horse is not dead, he sleeps that way." Indeed, sometimes the horse was lying on its back with its legs locked straight up in the air. How unusual was this behaviour and why or how did the horse learn to sleep that way?

Rosemary Bell, Bradford, West Yorkshire, UK

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