less is uncountable number
(i had less sand in my shoes then children when we returned from the shore, but still enough to make a sandy mess.)

(i put less sugar in my tea than my mother)

Fewer is countable.
we know how many bottles are sold, and how many recyled.

Fewer people attended the Harry Potter movie than expected. Grosses for Movie X were more than double that for Harry Potter.

the number of bottles or movie attendee's might be a large number, as is the number of bottles, but it is countable (and is counted, (beverage companies can tell you almost precicely how many bottles they sold (how many were dropped and exploded before sales, (stores can tell you how many where stolen..)

the numbers of grains of sand or sugar are not considered countable.. (how many grains of sugar are there in a spoonful? (does every spoonful contain the same about?)

my other obsession