From The Virtual Mirror (1994)

1. No known species of reindeer can fly. But, there are 3,000,000 species of living organisms yet to be classified, and while most of these are insects and germs, this does not completely rule out flying reindeer, which only Santa has ever seen.

2. There are two billion children (persons under 18) in the world. But since Santa doesn't (appear to) handle the Muslim, Hindu, Jewish and Buddhist children, that reduces the workload to 15% of the total - about 378m. At an average rate of 3.5 children per household, that's 91.8m homes. Presumably there's at least one good child in each.

3. Santa has 31 hours of Christmas to work with, thanks to the different time zones and the rotation of the Earth, assuming he travels east to west (which seems logical). This works out to 822.6 visits per second. This means that Santa's sleigh is moving at 650 miles per second, 3,000 times the speed of sound. For purposes of comparison, the fastest vehicle on earth, the Ulysses space probe, moves at a poky 27.4 miles per second, and a conventional reindeer can run tops 15 miles per hour.

4. The payload on the sleigh adds another interesting element. Assuming that each child gets nothing more than a medium-sized Lego set (two pounds), the sleigh is carrying 321,300 tons, not counting Santa, who is invariably described as overweight. On land, conventional reindeer can pull no more than 300 pounds. Even granting that "flying reindeer" could pull ten times the normal amount, they cannot do the job with eight, or even nine. Santa would need 214,200 reindeer. This increases the payload - not even counting the weight of the sleigh - to 353,430 tons. This is four times the weight of the Queen Elizabeth II.

5. Travelling at 650 miles per second 353,000 tons creates enormous air resistance - this will heat the reindeer up in the same fashion as spacecraft reentering the earth's atmosphere. The lead pair of reindeer will absorb 14.3 quintillion joules of energy. Per second. Each. In short, they will burst into flame almost instantaneously, exposing the reindeer behind them, and create deafening sonic booms in their wake.

6. The entire reindeer team will be vaporized within 4.26 thousandths of a second. Santa, meanwhile, will be subjected to centrifugal forces 17,500.06 times greater than gravity. A 250-pound Santa (which seems ludicrously slim) would be pinned to the back of his sleigh by 4,315,015 pounds of force.