Puzzle mechanics

Every scattered puzzle consists of five layers:

• A transparent plate of acrylic that serves as the base (15x15cm)
• 3 separate layers that consist of an outer frame and 20-ish puzzle pieces each.
• The individual layers are more or less nonsensical.
• Once placed on top of each other in the correct orientation, the overlapping pieces reveal the final image.   
• A second transparent plate closes the puzzle, so it does not have to be kept in its box. 
• You then attach the layers with some simple nuts and bolts. The final result will be wide enough to stand up on its own. 

Here's an example of the above Biohazard puzzle, and what the individual layers look like. If you're still unsure, you can check out the complete how-to on making your own on Instructables!