I first saw this puzzle when it was the monthly "Ponder This" challenge from IBM Research. If you like it, you might enjoy looking through their archives: http:

I first saw this puzzle when it was the monthly "Ponder This" challenge from IBM Research. If you like it, you might enjoy looking through their archives: http://domino.research.ibm.com/Comm/wwwr_ponder.nsf/pages/index.html http://domino.research.ibm.com/Comm/wwwr_ponder.nsf/pages/in...