Wikipedia: The Soma cube is a 3D puzzle invented by Danish polymath Piet Hein in 1933.  Seven different pieces made from unit cubes should be assembled into a 3×3×3 cube. The pieces can also be used to make a variety of other 3D shapes or figures.

The 7 piece set consists of six combinations of four-unit cubes, and a single three-unit cube.  Thus, 3 + (6 × 4) is 27, which is exactly the number of cells in a 3×3×3 cube. Of these seven combinations, two are mirror images of each other.