good puzzle, dean.

i think i got it. see comments at linked site.

I saw that one in Scientific American.
And before you say that I'm a geek, I'll point out that this in my dentist's waiting room. My dentist was a geek (I say "was" because he retired so he could do math. Now do you believe me?)

Good for your dentist! But next time you pick up a Scientific American -- don't sheepishly apologize "I only read it for the centerfold."

I'd seen it before too, so I declined to comment.