The Chinese Rings Puzzle
The Chinese rings (or baguenaudier) is one of the oldest known mechanical puzzles. The object of the game is to remove all of the rings from the horizontal loop of handle.[more]
Rules: the rings are interlocked so that only two basic types of move are possible.
1) The rightmost ring can be removed or replaced at any time;
2) Any other ring can be removed or replaced if and only if the ring to its right is on the handle and all rings to the right of that one are off.
The current state of ring can be represented in binary notation, writing 1 if a ring is on the handle and 0 if it is off. The solution uses the binary Gray code.[less]
 D. E. Knuth, The Art of Computer Programming, Vol. 4, New York: Addison-Wesley, 2001 pp. 5–6.
 H. L. Wu, The Three Classical Puzzle of China, Beijing: Science Press, 2004 pp. 143–186.