Some people may not even know that "fewer" and "less" cannot be switched in and out for each other in a sentence. "Fewer" or "few" are used in a sentence for things that can quantify. In other words, "fewer" is used for countable nouns: person, place, or thing. For example in the sentence, The girl has fewer than five dolls. One would use "fewer" in this sentence because dolls can be quantified.
One would use "less" in a sentence for things that can be hypothetically quantified. For instance, in the sentence, You should spend less of your time complaining. Time in this sentence is abstract and cannot be hypothetically quantified.