If you like this thing, please, please subscribe to the 3D Printing YouTube channel "Ethan Makes". The kid really needs your support, and his goal is to reach 500 subscribers:
http://goo.gl/ELT7ii

It's a puzzle box, based on a design by Osanori Yamamoto. It's called Identical Twins. The goal is to fit the two identical pieces inside of the base.

As a bonus, you can also print the Tetrahaedral (triangle shape) to create The Cell puzzle with the same base. Try to get the triangle shape in and out of the base through the small holes.

For a challenge, print the base as one piece. Otherwise, print as two pieces and join with glue. For a challenge, print the Tetrahaedral pointy side up (requires some great cooling). Otherwise, print pointy side down with supports.

While you're here, check out my other puzzle:
Pyramid Puzzle

If you can't solve this and want to give up, you can message me on Thingiverse, and I'll send you the link to the Youtube video with the solution.

Printing:
If you try to scale this model up or down, the tolerances will be affected.

Make sure that you print two of the Twin pieces.

Your printer must be capable of printing accurately on all axis in order for the puzzle pieces to fit properly. If you have a Wanhao I3, Monoprice Maker Select, or other cheap I3 clone, you may not be aware that your printer does not print in the Z axis properly as it comes from the factory. You need to fix your Z steps per mm or scale down the model on the Z axis to 98.7654%. If your first layer is too squished, you will probably need to trim off the brim. The pieces are intentionally snug.