What’s A Unit?

Today my daughter asked me what a calorie is.  I told her it was a unit of energy.  Then she asked me what a unit is.  I told her a unit is a single thing.  And she asked, “But what IS it?”  I was stumped.  So, I did what any parent would in this day and age, I looked it up on my phone.  This is what I found, “An individual thing or person regarded as single and complete but which can also form an individual component of a larger or more complex whole.”  Let’s just say that did no good at all.  I noticed another definition which seemed more apt to the original question, so I tried, “A quantity chosen as a standard in terms of which other quantities may be expressed.”  That was even less successful than the first definition.  I tried listing different types of units:  an inch is a unit of length, a pound is a unit of weight, etc.  That didn’t help much either.

I never did explain it in a way that satisfied her.  (Although I think she does get what a calorie is.)  Now I can’t help but wonder if I know what a unit is.  How can I claim to know something if I can’t pass that knowledge along to someone else?  It’s not like a unit is some amorphous thing like pornography.  According to the dictionary definition, it is an actual thing.  But it’s not a thing I can point at.

The more I’ve thought about it, the less satisfying the dictionary definitions are to me.  I don’t know if it’s a case of looking at the same thing for too long makes it blurry, but now these definitions sound like gobbledygook.  An individual thing that can form an individual component?  A quantity that expresses a quantity?  It’s not as bad as the definitions that use the word in the definition, “A unit is any thing that can act as a unit,” but it’s not much better.

I know I’ve spent way too much time thinking about this.  I’m sure my daughter has already forgotten about it.  But I’m going to be bothered until I can come up with a way to explain a unit that a kid can understand.  I need to find something that I can point to and say, “That.  That right there.  That’s a unit.”  Until then, I can’t help but think I simply don’t know the answer.

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