It is inspired by a clever puzzle I saw. It uses two metal spheres to keep the two halves closed, and you need to find the way to open it up.

The balls are 12mm in diameter and need to be metal in order to move smoothly and feel the mass position changing from the outside. They can also be found in the Bambu Store in a pack of 10; only 2 are needed.

It can be printed in any material, so I chose PLA; just make sure to print it strong enough not to break with little abuse of force (avoid Silk PLA).

HOW TO CLOSE IT

Just place the two balls in the two grooves on the central tower of the base and keep them in position with your fingers while sliding the lid on.

HOW TO SOLVE IT

I would leave you trying to solve with only some hints. In the closed state, the balls can move freely inside the donut-shaped passage. Placing it with the split vertical, you can move the balls using a lip on the inside of the lid. The balls must be placed inside the two grooves again in order to open it. Randomly moving the two halves, you will feel that sometimes they get stuck: it means one of them is inside its groove. Now the difficult part: keep it in place while placing the other ball in the other spot. 

GOOD LUCK!

If you need more help, just ask me or look for the solution online.