Designed and made with a 3D printer by Diniar Namdarian, 2018; sleeve added by J. Storer.
(plastic box 4.1 x 5.2 x 1/2 inches, 10 plastic pieces, and directions with solution pdfs)
The problem is to exchange the eyes. The circles slide on top of the pieces, where each have a raised portion, named "eyebrows" by the puzzle directions, that each can pass over everything except the opposite eyebrow. The precision afforded by this nice example of 3D printing provides for smooth sliding of the pieces.
In addition to allowing curved pieces, the solution that comes with the puzzle allows a piece (or a number of interlocking pieces) to push another piece (or a collection of interlocking pieces) as it moves; we refer to this as generalized rectilinear movement. For example, steps 16, 19, 25. 39, 49 of this solution move two pieces at once. Although this solution is presented as 65 generalized rectilinear moves, it appears that it can be improved to 59 generalized rectilinear moves, by combining steps 21+23, 26+27, 33+34, 44+45, 50+51, and 62+63.
At first, the puzzle looks to have a relatively simple solution that that exchanges an eye with the mouth, then exchanges the two eyes, and then exchanges the other eye with the mouth. Without the eyebrows this idea works, and a solution of 41 rectilinear moves (37 generalized rectilinear moves) is shown on the following three pages.
However, with the eyebrows, solving is more complicated. Following the three pages of the solution without eyebrows, is a 68 rectilinear moves (61 generalized rectilinear moves) solution.
Shorter solutions are possible by allowing the circles to be half on a piece and half hanging over empty space, but such movement is not allowed by the puzzle directions, and is not natural.
