Ever see the sign “20 items or less” in a supermarket checkout lane and gotten a strange inkling that it was wrong? Shouldn’t it be fewer?

Let’s see…

Fewer is a “count noun.” A count noun is something that can be counted or enumerated; such words can be made singular or plural. So, if the object can be counted, use “fewer”:

With the flu going around, fewer kids were at school.
The team suffered no fewer than four injuries in the sobering loss.
The fewer candy bars you eat, the fewer cavities you will suffer.

Kids at school, injuries, and candy bars all can be counted.

Less is a “mass noun.” Such nouns, like applesauce, can’t be counted because they’re an undifferentiated unit – you can count how many peas you eat but not any much applesauce you eat; they also cannot be made plural. So if the word is about bulk (and hence “uncountable”) and can only be singular, use less:
Children are playing less outdoors these days.
He showed less kindness that I expected to the homeless woman.
I‘ll need less than a gallon container of fruit punch for the birthday party.

Playing, kindness and fruit punch can be quantified.

Given this, that supermarket sign should say “20 or fewer items.” But don’t bring a marker to write the correction – we wouldn’t want you getting in trouble!