Meeting Point in a 3D Maze
Layers of a 3D maze are shown from top to bottom. You can move horizontally within a layer if there is no wall between compartments. You can move from a compartment with a white floor to the one immediately below it and back. The red and green points approach each other in each step. Where will they meet?
 M. Gardner, The Colossal Book of Short Puzzles and Problems (D. Richards, ed.), New York: W. W. Norton, 2006 pp. 280–281.