Sign at the checkout of a supermarket: “Ten items or less”. This is INCORRECT.
You can count the items, so you need to use the number word “fewer”. These nouns are countable.
e.g. : "Ten items or fewer." This is CORRECT.
If you can’t count the substance, then you should use “less”. These nouns are uncountable.
e.g. : "You should eat less meat." This is CORRECT.
Original idea from http://www3.telus.net