Introduction: The Impossible Puzzle: Made Possible by FDM Printing

I designed this puzzle using TinkerCAD after seeing something similar by Mr. Puzzle. Traditionally, a puzzle like this requires rather sophisticated woodworking skills. However, using principles of FDM 3D printing, anyone with a 3D printer can make their own. 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.

Step 1: Needed Supplies

PLA/ABS/PETG filament:I prefer PLA for a project like this:

An FDM 3D printer: I used the Monoprice Maker Select v2.1 ( but others will also work.

STL files:see the attached file or go to

A 100mm x 2mm steel rod ( cut into two 35mm long rods and one 5mm rod.

A 1mmx3mm neodymium magnet (

A spark plug: I designed it using the NGK (3951) TR55 V-Power Spark Plug (, but a similar sized spark plug will work.

Alternatively, I used 3 inch bolts/threaded rods with nuts and wingnuts instead of the spark plug. Other objects can be used too. The bottom of the object just needs to fit into the 5mm hole in the bottom of the cage and fit within the box.

Super glue. I prefer the gel, but the normal will do. The dollar stuff works great, but if you need an online option:

Step 2: Printing the Puzzle

Important!!! By reading further, you will know how the puzzle works and so solving it is no longer a mystery.

Print with 10 walls or print it completely solid. The 10 walls are needed to make sure the 5mm hole is fused with the bottom. Print the puzzle with the opening on the print bed as shown.

The steel rods are placed while the peice is being printed, literally becoming part of the object.

Pause the print at ~ layer 25 and again at ~ layer 196 (depending on the orientation of the object and the print layer heights). Both are just past the halfway point of the slots for the 2mm steel rods. One will also have the 3mm cavity for the magnet and 2x5mm rod.

As oriented in the Thingiverse stl file, layer 25 should have slots for one of the 35mm rods and the 5mm rod.

  1. Glue the magnet in first using a fast setting super glue.
  2. Place the 5mm rod into its slot
  3. Place the 35mm rod into its slot

Resume the print (NOTE: After placing both rods, monitor your printer closely to make sure it doesn't accidentally knock out a rod. Once the rods are covered by filament it should be good to continue as normal.)

At layer 196, pause the print again and place the second 35mm rod.

Resume the print (NOTE: After the rod, monitor your printer closely to make sure it doesn't accidentally knock out the rod. Once the rod is covered by filament it should be good to continue as normal.)

Step 3: Adding the Spark Plug to Complete the Impossible Puzzle

When it is done printing, the puzzle is in the locked position. To unlock, smack it down on a hard object to cause the smaller rod to fall from the magnet and enable the removal of one of the steel rods. See the YouTube video to see how this is accomplished.

Place the Spark Plug (or bolts or other objects) inside.

Replace the rod. If you accidentally tipped the puzzle and locked it again, you may to to slap it down to release the 5mm rod again so the 35mm rod can be replaced.

With the spark plug in place, you are now ready to watch your friends really puzzle over how you got it inside.

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, slide the free steel bar to the side and slip it out by pulling it away from the puzzle. The object inside can now be freed. Replace the steel bar to hide the secret.

Using the alternative bolts, it is possible to design it to that one of a pair can be removed without doing the trick. The other bolt, however, requires the 35mm rod to be removed first.

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