If you want to be a member of the Grammar Police, just grab a red pen and head down to your local supermarket. Once inside, look for a sign that says something like 10 Items or Less. 

What’s the error? It should be 10 Items or Fewer.

Always use fewer for quantifiable objects. As “items” can be counted, they are quantifiable. Because of this, you take fewer pretzels and fewer vacation days.

Use less when what you’re discussing what isn’t quantifiable or countable, when you’re talking about something that is nebulous or generalized. So to successfully diet, you would eat less junk food, and your stupid boss doesn’t care if vacation means less stress in your life.

On second thought, before going down to the supermarket to cross out less and write fewer in its place, first check your own writing for this mistake.