1. They do know the definition of exponents. It’s written a line above. They did it a line above.
2. They’re doing this with confidence. There aren’t erased numbers. This isn’t slow thinking. This is just what kids think seven squared ought to be.

By defining exponents in terms of multiplication while offering no other images or models for what exponentiation does, we create a default model for exponents that sticks with people forever. When mentally taxed — either with a tough multiplication, or with an unusual power — kids revert back to this default model. They’ll do this especially in high school, and they’ll get questions wrong on tests and all sorts of other things not because they’re being sloppy, but because this default model is constantly lurking in their minds.

Incidentally, I asked these kids why they think about multiplication when they see powers, and this is what they said:

I’ve written about a lot of this stuff before. See here, especially, where I shared the high school versions of this mistake.

Now that all of this has been established, the next step needs to be finding a curricular approach that doesn’t rely as heavily on the “repeated multiplication” model for exponents. We need to build a distinctive set of images and intuitions that are native to exponents so that our kids aren’t always defaulting into multiplication when they have to think hard about math.

This is work that I’ve started, in a post titled “Exponents Without Repeated Multiplication.”  I’ll send you there for the details, but I stake out two major claims about exponents education:

1. Much in the way that arrays support early multiplication work, geometric notions of area and volume can serve as the bedrock of an exponents education
2. We tend to think of four, not five, major operations of arithmetic, but we need to start thinking about exponents as on par with all the others and taking care to build them thoughtfully throughout the entire curriculum.

Beyond all of this, these exponents mistakes serve as a big reminder about the nature of learning, teaching and knowledge. The big, big lesson of all of this is that knowing/not-knowing is not clean and it’s not binary. There are degrees of knowing something. Would you say that these students don’t yet understand what exponents mean? What does that even mean, given the contradictory evidence we have in front of us.

But, then, what does it mean to understand something at all?

Here’s a short mistake that I came across today that I found interesting.

I was chatting with a 5th Grader. The question was, “What do you think is your top speed?”

Her: I don’t know how fast I run.

Me: Well, you know here is how fast I walk. [Walks.] I think that’s about 3 miles per hour.

Her: OK, well maybe I can run 6 miles an hour.

Other Kid: You can run way faster than that. You can run 15 miles an hour.

Her: Well, yeah, for a little bit. But I couldn’t run 15 miles in one hour. I’d get tired.

I don’t give enough thought to miles per hour. It’s really an abstraction of realistic rates, rates that you could actually use. Like, if it takes me 3.9 seconds, on average, to add a paperclip to a chain, then I can use that to realistically figure out how many paperclips I could chain together in 5 minutes. But miles per hour — at least in the context of running — isn’t realistic in that way. It’s a concept that imagines a world that pays attention to my current speed but strips away all the reality of exhaustion and physical limits.

In the future I’m going to try to be more sensitive and explicit about this when talking about miles per hour with little kids.

Thoughts about rates and the units we use would be very, very welcome. Share interesting anecdotes in the comments, please.

A nice mistake with the place value here, where kids are adding “.5” to the “.25.”

This is an awfully common mistake. What are some of the curricular approaches that help kids avoid these sorts of things?

(Thanks to Chris  for the submission!)

Thoughts?

(Thanks to Pam for the submission.)

What’s the relationship between division and square roots in students’ minds? Why did the kid write 15 / 15.

[Note: no idea how to categorize this in CCSS. Also, thanks to Timon for the submission.]

Question: Evaluate the expression $-z^{2} + x(3-y)^2$  when $x = 10$,   \$latex y = -2\$, and  \$latex z = -2\$.