So Happy Together #3

Make the marriage of your digital projector and laptop a happy one.

One Idea:

Become the teacher/learner hybrid.

Like this:

I buy some tea from the deli next to the school. $1.80. I’m at the grocery store the next day and notice a box of the same tea for $6.49. 18 bags. I start wondering, “how much is the deli profiting off our transaction?”


I take a picture of the box and put it in front of my beginning algebra classes the next day — today.

This is how we introduce rates: how it doesn’t just matter that the box costs more or that you get more tea in it rather you must consider the two things against each other.

After we divide price by bags and come up with 36 cents per bag, the students notice that same figure on the price tag. The grocery store helpfully makes these calculations for you.

So when I put up two more photos, two sports drinks, one in bulk, the other in miniature six-pack, I censor those figuresIn this particularly mind-blowing example, the cost per ounce was the same..

And we spend 45 minutes running computations, discussing the results, arguing over the significance of the results — all from three photos.

No wipes, checkerboards, animations, or other PowerPoint detritus. This is the 21st-century digital projector bashed back into the 20th century.

This is one of those carousel slide projectors I’ll only pretend to have seen in person. I’m only projecting still images but here, in the 21st century, I can draw those images from infinite sources from around the globe or my local supermarket.

This practice habituates like a hard drug. With a digital projector waiting back in my classroom, I can’t help looking for interesting, relevant images to put in front of my class.

A digital projector shrinks the time-gap between my learning moment and theirs.

A digital projector has effectively buried the difference between What Fascinates Me and What I Teach.

Previous Editions:

I'm Dan and this is my blog. I'm a former high school math teacher and current head of teaching at Desmos. More here.


  1. Thanks for the incredibly easy way to use a projector in class. I will be sharing this with the teachers on my campus to show them just how easy all this digital stuff can be!

  2. This is so great because kids will learn from real world examples using money. I remember being terrible at math as a kid, except for the times the teacher put dollar signs in front of the numbers… then I was her best student. What a difference a small change in perspective makes.

  3. Great use of digital photos to introduce rate. I tried a similar thing this year. My problem is steering the discussion to an understanding that “rate” is a way of saying “how much for one”, and then tying that to “slope” represents how much the graph goes up every time you go over by one. I feel like there is not much help or training available to develop that skill as a teacher.