ABSTRACT
One of five students, called by the names of the weekdays, is to have a test. The one designated has a gold star pinned on her back while the others have silver stars. The teacher tells them the one chosen will not know in advance that she has been selected for the test. Monday to Friday are lined up in order one behind the other, with Friday at the back. Each student can see the backs of just those further up the line: Friday can see the backs of all the others, Thursday can see the backs of Wednesday, Tuesday and Monday, and so on. Everyone knows it can’t be Friday, since Friday would see that none of those in front have the gold star. But then they know it can’t be Thursday either, since Thursday knows Friday is out and none of the others had the gold star on their backs. And so on for Wednesday, Tuesday and Monday.
