Santa delivers the goods thanks to shrinking act... and a little bit of magic
The mystery of how Santa can deliver presents to 700 million children in one night, fit down the chimney and arrive without being seen or heard has been 'solved' by a physicist.
Santa and his reindeer travel the world at such speed that - according to Einstein's special theory of relativity - they shrink, enabling him and a huge sack of presents to squeeze through even the tightest of chimneys.
Dr Katy Sheen, a physicist at the UK's Exeter University, also found a scientific explanation for why Santa is so rarely seen by children, even though millions stay awake on Christmas Eve hoping to glimpse his red suit.
The physicist calculated that Santa would need to travel at about 9.9million kmh to deliver presents to every child expected to celebrate Christmas in 31 hours - taking into account world time zones.
Such speed would make him change from red to green and, at greater speeds, eventually disappear entirely. In this case, it is the Doppler effect which makes Santa change colour because the light waves bouncing off him get squashed.
The Doppler effect also explains why children cannot hear him arrive. As Santa and his sleigh approach, the sound of bells and his deep "ho, ho, ho" would get higher and higher in pitch and then become completely silent, as his voice moves beyond human hearing range.
If children hear a bang on Christmas night it could be Santa's reindeer breaking the speed of sound, resulting in a 'sonic boom'.
"Some strange things happen when you start to travel that fast. Firstly, time slows down. Second, Santa gets squished, which means that he can fit down a chimney more easily," Ms Sheen said.
"For Santa to fly this fast it takes lots and lots of energy. How does Santa manage to reach these phenomenal speeds? Well that's magic. However, he would certainly need a lot of fuel."