Perhaps a good intuition pump is this: after every guess the person claiming to have found Waldo gets to position three things: the book, the waldo mask, and th

Perhaps a good intuition pump is this: after every guess the person claiming to have found Waldo gets to position three things: the book, the waldo mask, and the covering cardboard. This then means the subsequent trials don't reveal any side channels about Waldo's location in the book. (And of course in practice the "pieces of cardboard" are big enough to obscure all book information.)