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Ken Bloom | USLHC | USA

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An Elmo mystery

While we were visiting my family back east a couple of weeks ago, one of our relatives gave my two-year old a present — a book featuring Elmo and a talking pen.  Now, my wife and I are no fans of licensed merchandise, not to mention electronic toys that talk.  But this book — which comes from an outfit called Follow the Reader — turned out to be a bit of a physics puzzle.  (You might want to try some of the demos on their site as I describe it.)

Here’s how it works — on each page, there is an arrow that serves to initialize the pen for that page.  You slide the pen along the arrow, and then Elmo says something appropriate for the pictures on that page.  Then there are other arrows or even just patches of color that you slide the pen over or touch the pen to (respectively), and then Elmo says something appropriate for that part of the picture.  (Trust me, this is easier to follow if you have one of the books.)  My daughter was pretty intrigued just by the talking part, but everyone else in the family was pretty puzzled by what the underlying technology was.

These were some of the experimental observations that we made:

  • There are only four unique “signals” that the pen could pick up.  On any given page, you could get a maximum of four different responses out of Elmo, depending on where you put the pen.
  • Better still, you could initialize the pen for one page, and then use it on a different page, and get responses appropriate to the first page.
  • The regions of the pages that activated Elmo were of many different colors…but the previous point demonstrates that the visible color isn’t relevant to the response.
  • If you slide the pen the wrong way along the initializing arrow, you could sometimes get Elmo to talk as if he were initialized for a different page.
  • You can see through the pages if you hold them up to the light, and you can’t produce the effect you get on one side of the page by applying the pen to the other side of the page.

I ended up taking the book to campus so that I could show my colleagues how it worked, and we started batting around some hypotheses.  With a little bit of lab work (non-destructive, my daugter will be happy to know), I think we’ve been able to crack the puzzle.

So what, dear reader, do you think is going on here?  I invite you to speculate in comments, and I’ll post what I think the answer is in a few days.  Apologies to all you LHC fans for going a bit off topic, but hey, this is what science is all about!

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