A few years ago, when I was visiting my (very) old friend Charlie up in Washington, he gave me what has become a treasured possession.
It looks like a normal, old, wooden ruler. A foot long with inches on one side and centimeters (going the other direction, of course) on the other.
But feast your eyes on it. If you don’t see the problem immediately, that’s normal. Just relax. Take your time. And wonder: how did this happen?
Please don’t give it away in the comments. Be sly 🙂
A Best-Case Scenario 
I thought metric was a 10 based system …
This reminds me of working with 2nd graders on figuring out “how far is it from 2 to 8?” They asked things like, “Do I count all the numbers starting with 2 and going through 8?” (2,3,4,5,6,7,8 would mean the answer is 7) Do I just count the numbers in between 2 and 8? (that could give an answer of 5). The real clue is that you count the jumps. That’s what the problem is on the ruler.
Hi Jan! What an intriguing take on this. Not the way I think about it — though I agree the jumps-versus-spaces issue trips up even adults. We see something similar when students line up the “1” to start when they measure.
Okay, all: in case you haven’t seen this, watch Cliff Stoll about a related homework problem: https://www.youtube.com/watch?v=9yUZTTLpDtk&t=3s