My son was asked this question in third grade.  I thought it was 999,
but apparently I am wrong.  Can someone explain this one, please.

Dear jresq,

One million is defined as 1000 x 1000, one thousand thousands. The
word "million" reflects this since it is derived from the Latin
"mille", which means "thousand". See also these websites:

Wikipedia Online Encyclopedia: Million
http://en.wikipedia.org/wiki/Million

Bartleby Dictionary: Million
http://www.bartleby.com/61/31/M0303100.html

999 x 1000 would be only nine hundred ninety-nine thousand (999,000).

Regards,
Scriptor

Thanks, you made it clear.

Yes but in Britain is not a billion a million million i.e
1,000,000,000,000 but in US it is a thousand million as in
1000,000,000. Or have they both agreed on what constitutes a billion
now?? In Australia we use the US defintion. In fact we are most
confusing of all as we mix & match and borrow from both Britian and US
in both words and their meanings,  and numbers
  Interesting thought that 999,000 was a million. Never heard that
view before. On that basis 999 would be a thousand and 99 would  be
100, and then of course 9  would be actually 10. In reverse we would
have 10 as being 11, 100 as 101 etc.