This is a wooden puzzle. The two parts lock together and you give it to a friend, asking him to unlock and separate the two parts. A friend in Germany made this one. When he handed it to me to solve, I could not until he showed me the secret. This Instructable will not only show how the puzzle is solved, but will also show how you can make these to give as gifts, especially at Christmas.
I have learned through a link by Make! Magazine that this is called a Duallock Cross Puzzle and it was invented by Nobuyuki Yoshigahara.
Step 1: How the parts appear separated
Here you see the two halves of the puzzle separated. Each half has a center notch and a hole below it. Internally, there are two holes in each with a piece of dowel pin that can move freely in each hole. The dowel pins are each a bit longer than the center notch. The center notch is the width of the wood's thickness on the parts. I will use some 3/4 inch clear pine.
Step 2: Readying the puzzle to be solved
The person who solves the puzzle should see you assembling it. He should know about the two dowel pins. To assemble the parts, push the dowel pins back into their holes. Invert one half of the puzzle and let the parts settle into the two notches.
Step 3: Tumble
Tumble the assembly back and forth so the dowel pins move through the holes in the mating parts. Hand it to a friend and ask him to solve the puzzle. If he has seen this before, or is very smart, he will solve it right away. If he has not seen it before, he will struggle with it until you show him how it is solved.
Step 4: The solution
I hinted at the solution in the title of this Instructable. Place the locked puzzle on a flat surface. With your finger and thumb, spin the puzzle as rapidly as you can. This creates a centrifugal force that flings the dowel pins outward from the center and releases the two pieces so one can easily be lifted from the other.