I always liked the fact that 10! (10 factorial) is exactly the number of seconds in six weeks.

  6 weeks * 7 days * 24 hours * 60 minutes * 60 seconds:

    6 * 7 * 24 * 60 * 60
  = 6 * 7 * (3 * 8) * (4 * 5 * 3) * (3 * 2 * 10)
  = 6 * 7 * 3 * 8 * 4 * 5 * (3 * 3) * 2 * 10
  = 6 * 7 * 3 * 8 * 4 * 5 * 9 * 2 * 10
  = 1 * 2 * 3 * 4 * 5 * 6 * 7 * 8 * 9 * 10