Is There a Santa Claus?
1) No known species of reindeer can fly. BUT there
are 300,000
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 2 billion children (persons under 18) in the
world.
BUT since Santa doesn't (appear) to handle most Muslim, Hindu,
and Buddhist children, that reduces the workload to 15% of the
total  378 million according to Population Reference Bureau.
At an average (census) rate of 3.5 children per household, that's
91.8 million homes. One presumes 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 is to say that for each Christian
household with good children, Santa has 1/1000th of a second to
park,
hop out of the sleigh,
jump down the chimney,
fill the stockings,
distribute the remaining presents under the tree,
eat whatever snacks have been left,
get back up the chimney,
get back into the sleigh and
move on to the next house.
Assuming that each of these 91.8 million stops are evenly
distributed around the earth (which, of course,
we know to be false but for the purposes of our calculations we
will accept), we are now talking about .78 miles per household, a
total trip of 751/2 million miles, not counting stops to do what
most of us must do at least once every 31 hours, plus feeding
and etc.
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 man made vehicle on earth, the Ulysses
space probe, moves at a poky 27.4 miles per second  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 mediumsized lego
set (2 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" (see point #1) could pull TEN
TIMES the normal amount, we cannot do the job with eight, or
even
nine. We need 214,200 reindeer. This increases the payload

not even counting the weight of the sleigh  to 353,430 tons.
Again, for comparison, this is four times the weight of the cruise
ship Queen Elizabeth II.
5) 353,000 tons traveling at 650 miles per second creates
enormous air resistance  this will heat the reindeer up in the
same fashion as a 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. The entire reindeer
team will be vaporized within 4.26 thousandths of a second. Santa,
meanwhile, will be subjected to acceleration forces 17,500.06 times
greater than gravity. A 250pound Santa (which seems ludicrously
slim) would be pinned to the back of his sleigh by 4,315,015 pounds
of force.
In conclusion 
If Santa ever DID deliver presents on Christmas Eve, he's dead now.