This puzzle will test your spatial ability. The goal is to remove the tetrahedron (triangular pyramid) from the cage. It's really pretty straight forward if you paid attention in geometry.
Solution Video: https://youtu.be/t2j4Z-3CLDc
Nice Wood Version: http://amzn.to/2ztacMh
The green version was printed 1:1. The purple version was scaled 50% larger. Note that I used a low end 3D printer for these parts which didn't hold the dimensions very well. With that, I needed to sand down the opening in the purple version by ~ 0.03" to allow the tetra to fit inside. If this happens to you, I suggest reprinting the tetra at a slightly smaller scale.
More Info: https://www.instructables.com/id/Caged-Tetrahedron-Puzzle/