Where The Time Goes

So you’re like: “What do you guys know about Forever Stamps?”

You ask, with a grin that can only be described as money-grubbing, “Are you guys thinking what I’m thinking?

One of your students almost certainly is.

You set the up the scheme.

Depending on your grade level and calculator availability, you a) have ’em derive the equation or b) just show ’em that these things are predictable and that the price is rising.

You run the calculations and you build that sucker up. “I’m about to make all of you filthy rich,” you say, with your finger ready to drop like a hammer on the wireless remote.

“Ninety freaking dollars!”

“220% return on investment!”

“In only 25 years! Tell your parents! It’s better than stocks!”


And then you let them tear down your house of cards with a flurry of objections, including a) the inevitable total teeter-tottering of personal communication from postal service to e-mail, b) the difficulty of storing 100 stamps in mint condition over c) 25 years for d) a paltry reward which at that time won’t be worth but a few Twizzlers.

You let ’em tear you down because a) your retirement’s locked up in sturdier investments, and b) these discussions, the kind stirred up by reckless mathematics, are the last pin holding this thing together as we clatter towards the finish line where, if I have my way, we’ll still be finding math in odd places.I swear after a year with this projector/laptop/Keynote triple threat, it’s like every fifteenth RSS feed pops out at me demanding an audience with my kids.

This slide deck took an extra hour to cook up but, man, such a worthwhile investment. More than any other year I want to work this one straight through to the last bell of the last day. I can’t figure out a finer finish.


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


  1. Steve Peters

    May 15, 2007 - 7:19 pm -

    That’s the great thing about extrapolating with exponential curve fits. Hypothetical riches!

  2. Ha! I know, right? The effect just isn’t quite as powerful with a linear regression, which’d probably fit better, but where it’s like, “you made $6 … *thbbbpt* “

  3. I’ve read this post 5 times today, and each time debated whether or not to comment. I love exponential growth as much as the next guy, but the fit of this curve is really bothering me. You’re right; a linear regression would work much better, but of course that’s not as interesting. There’s really no need to comment on this. I just wanted to get it out of my system and off my mind.

  4. Yeah, glad you did. And it’s worth debating, I suppose. This is a class that came in pretty uninterested in mathematics and I decided to play fast and loose with the hard mathematics for the sake of dazzling them a bit. You could call it an error in judgment but it feels subjective to me.

  5. I agree — that data doesn’t look exponential to me. Who knows what was going on in the 1980s. I was busy with elementary school and didn’t mail a lot of letters myself. But I support you using that curve… they’ll see it again.

    Plus, it’s just not as easy to get kids riled up about Abraham Lincoln’s use of exponential growth in his 1862 Annual Message to Congress. They’re like, yeah, yeah, Civil War, slavery, racism, reckless projections based on census data, government schemes to get rich tomorrow, and no one’s doing the math… what else is new.

    But you start talking about stamps, and damn: that’s money. Am I the only one here that collected stamps as a kid?

  6. I don’t understand math too well, but what about short term investment? Buying a ton of stamps (say 10K worth) a week before the price goes up, and then selling ’em at current face value (that is, the new higher price – 1-3 cents or so above what you paid, no?) You wouldn’t make much, but it’d be something. Enough to buy, like, a good meal and six of hef, right?

    Of course, this is hypothetical: who would want to buy stamps at face value from some random math teacher?

  7. Right, there are lots of problems here. Also, according to that Slate article, there is a law on the books that prevents the cost of postage from rising above inflation. So even if you can get someone to buy your stamps at face value, you’re a loser.

    Of course, as Steve said originally, it’s tough to beat hypothetical riches as a classroom activity.