The mystery man – now revealed as South Elmsall artist Billy Johnson – left 57 of the foot-high carved stones on doorsteps across the country while conducting an experiment, along with a hand-written rhyme "Twinkle, twinkle, like a star. Does love flourish from afar?"

Mr Johnson sparked a nationwide hunt after being captured on CCTV placing three outside a post office in Braithwell, near Rotherham, with further finds in a Selby village, Goathland, near Whitby, Kilburn, near Thirsk and Arthington, near Leeds. He had wanted to follow their progress and see people's reactions at receiving a free gift.

George "Griff" Griffiths, an artist from Arthington, received three heads on two separate occasions.

During his research he visited York Minster, where stone masons examined a carving that appears to spell out the word "paradox", and searched the Internet for clues about the riddle – all to no avail.

Judi Alston, director of One to One Productions – the company behind Mr Johnson's website www.paradox-carving.co.uk – said: "Billy was taking part in an art project to follow the progress of these heads and see if they would take on a life of their own. It was not a publicity stunt – he isn't even aware of the huge interest this has caused."

Have you received a carved stone head? Contact the Selby Times newsdesk on 01757 702198.