It doesn't make sense to say: 2 apples + 3 apples = 5 apples This confuses a lot of people when they later learn algebra and believe that it is equivalent to:

It doesn't make sense to say: 2 apples + 3 apples = 5 apples This confuses a lot of people when they later learn algebra and believe that it is equivalent to: 2a + 3a = 5a Both of these equations look correct at face value but the critical difference is that an apple is not a variable. Do we mean the weight of the apple? Or the price of the apple? Or its energy content? Apple is too concrete to be used as a variable and it's not quantifiable. The word 'apples' invites people to think of the variable as a concrete object instead of a placeholder for a quantifiable property of an object.