A serious grammar discussion today!

Folks, this is about countable and uncountable nouns.  I'm getting really tired of hearing and seeing inappropriate uses of the word "less" when it should be fewer.   The one that bothered me most recently was a television commercial for a L'Oréal product that's supposed to help you get rid of dark spots on your facial skin.  It promises that you'll see "less dark spots over time."  You know, if I'm going to pay a lot of money for expensive French remedies for my skin ailments, you would think they could use of my purchase price to sponsor ad writers that know the difference between countable and uncountable nouns!  Here it is with a more formal explanation.

Countable nouns: any noun that you count indvidually
  Example: potato (one potato, two potatoes, three potatoes, etc.)

Uncountable nouns: any noun cannot be counted
   Example: justice (here, you cannot say one justice, two justices, etc.)

Now, when you use nouns in comparison with each other, you have to decide whether to use "fewer" or "less". When you are using countable nouns, you need to use "fewer" and when using uncountable nouns, you need to use "less".

For example, which is correct?

There were less goblins under my bed last night.
or
There were fewer goblins under my bed last night.

Since a goblin is a countable noun, the second sentence is the correct one.

Here's another example, and guess which one is correct:

I have seen less sadness in her since she started playing soccer.
or
I have seen fewer sadness in her since she started playing soccer.

I think this one is more obvious because it sounds really funny to say "fewer sadness."  Sadness is an uncountable noun, so "less" should be used in this sentence.

Knowing the difference between these is useful for using the words "amount" and "number", but I think I'll leave that for another day.