1) No known arts of reindeers can fly. But there are over 300.000 living organismes to be classified, and even though most of them are bugs and bacterias, it donīt exclude complitely the flying reindeers that only Santa has seen.

2) There are 2 billion children (people under the age of 18) on Earth. But since Santa doesnīt serve muslims, hindus, jewish and buddistic religions, his heavy work is reduced to only 15% of the children, - 378 millions.

An average by 3.5 children a household, it will count as 91.8 millions of children. One can presume there is at least one child in each home that has behaved during the year.

3) Santa has 31 hours during Christmas to work on, due to the different time zones anddue to the rotaion of the Planet Earth, assuming he travels from East to Weast (which seems more logical) This means Santa visits 822.6 homes per second. So for each christian household with kind children, Santa has 1/1000 second to park, jump off the sledge, get through the chimney, fill the stockings with presents, leave the rest of the presents under the Christmas tree, eat the candies maybe left, get himself up through the chimney again, back to the sledge and straight ahead to the next house.
Supppose that each of these household are equal placed on Earth (which we know, of course, is not true, but for this equation letīs say itīs true), and that there are at least 1.2 km between every household, a total trip of 120.8 km, not including stops for doing what we all have to do at least once every 31th hour, and feeding  the reindeers ans so on. It means that Santaīs sledge is moving with a speed of 1045 km/hour, - an ordinary reindeer can run for, most, 24 km/hour.

4) Letīs say every child would get nothing else but a bunch of Lego (approx. 1.5 kg) which would represent a total of 321000 tons to be carried on the sledge. Not including Santa himself, often described as overweight. 
On the ground a reindeer can pull no more than 220 kg. Even if one agree that "Flying reindeers"(see p.1) can pull 10 times the normal weight, it is not enough with eight reindeers. Santa would need 214.200 reindeers to deliver all the gifts! This would increase the weight of the sledge - to 353.430 tons again, for comparing. (Just for a comparison) that is four times the tonnage of the ship Queen Elizabeth.

5) 353 000 tons travelling for 1045 km per second creates a huge resistance in the air. It will warm up the rain deers the same way as space ships returning to Earth athmosphere. The leading reindeers would absorbe 14-3 quintiljona joule of poor energy per second, each. They would be on fire almost immediately, and exposing the back coming reindeers and creating a tremendous sound explosion. The whole group of reindeers will vanish in a period of 4.26 thousand parts of a second. Santa will in the meantime be exposed for the centrifuge power 17.500 times gravity. A Santa with the weight of 182 kg will be pushed to the back wall of the sledge with the power of 3.149.961 kg.

The conclusion must be, - if Santa ever delivered presents for Christmas, he must be dead by now. Still believe in Santa?
My translation from a Swedish text by unknown author

To children reading this page: Of course there is a Santa!
This is a joke to grown ups who have scientific minds! :o)