^Your awareness of numbers is split in the brain. We instantly
   know 1, 2, 3 or 4. In babies they've confirmed it to 3 (back in
   1916). Similar tests in animals have similar results. We know it
   even before we're aware of it knowing it. (subitizing) It's part
   of our Object Tracking System.

   Anything behind the number 4? Handled by a totally different
   part of the brain. Called "Approximate Number System", it's our
   ability to measure "bigger" and "smaller", "more" or "less". Our
   accuracy improves with experience. By adulthood, we easily
   distinguish 100 items versus 115 without needing to count.

   Cool, eh? Oh, and are able to work with small numbers better
   when they're on our left side, and better with larger numbers on
   our right side.^