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.