Tails Up: (E. Hordern, CFF 43, 1997) --------- A coin problem from a recent article in Quark (a Japanese publication). Start and end with the four coins positioned as the figure. A move consists in one coin jumping one or more others: up, down, left, right and/or diagonally. There must be a space to land on after each coin jumped. Every time a coin is jumped, turn it over. Starting with all the coins "heads up", reverse all the coins in five moves - "tails up". The puzzle is not nearly so easy as it looks. . . . . . . . O O . . O O . . . . . . . the 4 coins in the starting position Hint1: - as each coin jumps only an even number of row or columns we have the pattern: A B A B A C D C D C A B A B A C D C D C Each coin can never leave its class (A, B, C, or D). As we have a coin in each class, each coin must finish in its original position. The only tetromino with the same distribution of class is the 2-square. Hint2: - Try to reverse all coins in four moves for the configuration: . . . . . o o . . o o . . . . . Hint3: - Conjugate the 2-square solution to solve the original problem.