Kongming lock is a traditional Chinese intellectual toy. It is said that the toy was invented by Zhuge Kongming some 2,000 years ago based on the principles of the eight-diagram metaphysics. It was widely popular among the folks. The concave and convex parts inside the three-dimensional interpolation toy mesh perfectly. Kongming locks come in different shapes and sizes, with different internal structures. But they are usually easy to take apart and difficult to reassemble.
Here's another beautiful bit of wooden puzzle design and craftsmanship from Denver metagrobologist Kagen Schaefer. The locking mechanism was recently reverse-engineered by Instructables user dombeef, who has made a working paper version.