The mystery of how Father Christmas can deliver presents to 700 million children in one night, fit into a chimney and arrive without being seen or heard has been "solved" by a physicist at the University of Exeter.
Dr Katy Sheen, from the geography department at the British college, has theorised that Santa Claus and his reindeer zoom around the world at such high speeds that - according to relativity theory - they would shrink, enabling Father Christmas and a huge sack of presents to fit down chimneys.
She added that the Doppler effect - a change in frequency of sound, light or other waves often illustrated through the marked change in pitch of a fast approaching and departing siren - also explains why children cannot hear him arrive.
Dr Sheen explained her theory, which was prepared with the festive spirit in mind and not for submission to a scientific journal, to children at the University of Exeter last week to interest children in science and physics.
She calculated that Santa Claus and his reindeer would have to travel at about 10 million kmh - more than 200,000 times faster than Usain Bolt, the world's fastest man - to deliver presents to every child expected to celebrate Christmas in a span of 31 hours (taking into account world time zones).
Such speeds would make him change from red to green and, at greater speeds, he would disappear.
Children would not be able to recognise him as he would appear as a rainbow-coloured blur, eventually disappearing to the human eye, she said in a statement from the university.
At such speeds, Father Christmas would also shrink - or get thinner - in the direction in which he was travelling, allowing him to fit down a chimney.
Also, as Santa Claus and his sleigh approach, the sound of bells and his deep "ho, ho, ho" would get higher and higher (like when an ambulance siren whizzes by) and then become completely silent, because he would move beyond human hearing range.
"How does Santa Claus manage to reach these phenomenal speeds? Well, that's magic! However, he would certainly need a lot of fuel - so don't forget his glass of sherry, a mince pie or two, and some carrots for the reindeer!" Dr Sheen said.