Saturday, September 28, 2013

This Lesson Cost Me $1

Zero product property today, only the students don't know that yet.

For them, its a game-show-style guess-the-number game.

I want a fun prize for the winner, but all I can come up with is a couple of quarters. That will have to do.

Before class, I write a bunch of numbers on this board and cover them with index cards. I'll even give a hint about the first two.

The students are totally into it . . . 2! . . . 3! . . . 6! . . . 1! . . . 

Someone decides to try negatives . . .  -1! . . . -2! 

Finally, someone else tries 0.5. . . Fancy. But wrong.

I let this go on for a bit.

Then I break the bad news. Sorry guys. It was 6000 and 1/1000. Better luck in round 2.

And there are more guesses . . . 1! . . .  -1!  . . . 12!

Someone is on to me . . . "Guys, this could be ANYTHING!" 

So you give up? It was 58 and 1/58. Okay, on to round 3.

At this point I am expecting all hands to go up. In a perfect world, everyone would want to guess zero! Right? Wrong.

That's where I am surprised. One lonely hand goes up . . . Zero? He asks hesitantly. My first two examples raised enough skepticism that students are sure there must be a catch.

This isn't how it worked in my head but that is okay. I can adjust.

Is Tyler right? Can we know for sure that one of the numbers is zero? Discuss at your tables. 

I walk around and listen and most seem to be figuring it out. Someone suggests 5 and -5 but quickly realizes that won't work. For those who aren't convinced, I challenge them to come up with a number other than zero that will work.

We conclude that Tyler is right and move on to the final round.


 Not exactly. I tricked you this time by using variables. But tell me what you know . . . 

"x minus 3 or x plus 2 equals zero". 

Yep. That is all.

P.S. Next time I am giving everyone a white board to write down their guesses for each round. I had a lot of participation, but definitely regret that I didn't get a response from every single student.


  1. What a great idea! I wonder how to adapt it to grammar????

    1. If anyone can figure that out, it is you Mrs. E!

  2. I loved the first question the most actually. I think I'm going to use this to intro a couple properties in my Honors class, but questions like that first one would be great for some place value/powers of 10 stuff. We're going into some number sense in Algebra 1 next and I am going to try it with that as well. Great idea, thanks!

    1. I am happy that you can take this little idea and adapt it for your classroom. Thanks for reading!

  3. This is a great idea for teaching other properties too! I can't wait to try it with my students. I have one of those fancy SMART boards, that I honestly don't use for much besides a projection screen for my document camera. But this idea is rich enough mathematically to do a little prep and set up on the SMART board. I think having the students write their guesses on a whiteboard is a great idea. I also think I will have the students "Turn and Talk" to their partner about why those chose the numbers they chose to get some justifying worked in there. Thanks for the great idea!

    1. Hi Diana, thanks for reading! I hadn't thought about using this for other properties but of course you could. Good luck using this in your classroom!