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#### 一就是一...的确是吗？

One is one ... or is it?

(音乐)
(Music)

Which is correct — a dozen eggs is? Or a dozen eggs are?

I remember being in elementary school,

and my teachers making a big deal about the unit.

And I never really got that, until one day I was in the grocery store,

and I wanted to buy an apple, but I couldn’t buy one apple.

So I did. I bought one bag of apples,

I took it home, I took one apple out of the bag, and I cut it up.

And then I ate one slice.

One bag, one apple, one slice.

Which of these is the real one?

Well, they all are, of course, and that’s what my elementary teachers were trying to tell me.

Because this is the important idea behind

whole number place value, decimal place value and fractions.

Our whole number system depends on being able to change what we count as one.

Our whole number system depends on being able to change units.

There are two ways to change units.

We can compose, and we can partition.

When we compose units,

we take a bunch of things and we put them together to make a bigger thing,

like a dozen eggs.

We take 12 eggs, put them together to make a group,

and we call that group a dozen.

A dozen eggs is a composed unit.

Other examples of composed units include

a deck of cards, a pair of shoes, a jazz quartet

and, of course, Barbie and Ken make a couple.

That’s not a composed unit,

because we don’t get a whole bunch of slices from a whole bunch of different bakeries

and put them together to make a loaf.

called slices, so each slice of bread is a partitioned unit.

Other examples of partitioned units include

a square of a chocolate bar, a section of an orange

and a slice of pizza.

The important thing about units is that once we’ve made a new unit,

we can treat it just like we did the old unit.

We can compose composed units, and we can partition partitioned units.

They come in packs of two,

and then those packs get put together in sets of four

to make a box.

So when I buy one box of toaster pastries,

「四个」？还是「八个」？
four things, or eight things?

It depends on the unit.

One box, four packs, eight pastries.

And when I share a slice of pizza with a friend,

we have to cut it into two smaller pieces.

So a box of toaster pastries

is composed of composed units,

and when I split a slice of pizza,

I’m partitioning a partitioned unit.

But what does that have to do with math?

In math, everything is certain.
2+2=4 1就是1
Two plus two equals four, and one is just one.

But that’s not really right.
1不一定永远是1
One isn’t always one.

Here’s why: We start counting at one,

and we count up to nine — one, two, three, four, five, six, seven, eight, nine.

And then we get to 10, and in order to write 10,

we write a one and a zero.

That one means that we have one group,

and the zero helps us remember that it means one group, not one thing.

But 10, just like one,

just like a dozen eggs, just like an egg,
10是一个单位
10 is a unit.

And 10 tens make 100.

So when I think about 100, it’s like the box of toaster pastries.
100是一个东西？
Is 100 one thing,
10个？
10 things

or 100 things?

And that depends on what one is,

it depends on what the unit is.

So think about all the times in math when you write the number one.

No matter what place that one is in,

no matter how many things that one represents,

one is.