I designed this puzzle after seeing something similar by Mr. Puzzle. By 3D printing the puzzle you'll know the secret mechanism that makes it possible so it's no longer really a puzzle for the maker. However, its sure to stump your family and friends. Unlike the wooden versions that have glued joints, FDM printing makes the piece seamless and a real puzzle to those who do not understand what's happening inside.
I designed the holding compartment (cage) to either hold a spark plug (https://amzn.to/2slQRGe) or a 1/8" (5-32/6-32) x 3" bolt/threaded rod (https://amzn.to/2IXsJFb or https://www.lowes.com/pd/Hillman-6-to-32-x-3-in-Phillips-Slotted-Combination-Drive-Zinc-Plated-Machine-Screws-5-Count/3035868 or https://www.lowes.com/pd/Hillman-1-8-in-x-3-in-Standard-SAE-Threaded-Rod/3127739). A 3" x 1/8" Toggle Bolt would also work if you have those lying around. Of course metric equivalents are effective too. Other objects could also work. Add some nuts or wing nuts to the bolts for added effect - the wingnut makes for a very interesting puzzle with the threaded rod.
The bottom of the object just needs to fit into the 5mm hole in the bottom of the cage. Two 2mm steel rods (https://amzn.to/2H3PZeT) keep the object trapped inside. Each one is 35mm in length.
The secret is the small 2mmx5mm steel rod secreted inside, held in place by a 1mmx3mm neodymium magnet (https://amzn.to/2IX71B2).
The steel rods are placed while the peice is being printed, leterally becoming part of the object. Details are in the printing section.
To solve the puzzle (ie release the spark plug or bolts), simple knock the puzzle against your hand or hard object, causing the 5mm steel rod to fall down. Without tipping puzzle, the free steel bar to the side and slip it out by pulling it away from the puzzle while sliding back again. The object inside can now be freed. For more details check out the video: https://youtu.be/dKaclSiJrHg