A set of seven wooden blocks. Each piece is characterized not by its color or external shape, but by a concave.
The set consists of three shapes: two pieces in their original cubic form, four pieces with the corners chipped off in the shape of a quarter of a sphere, and one rectangular piece with a semicircle cutout. As the cubes are stacked, not only do the outer contours of the cubes change, but also the inner outlines start to create voids. The pieces can be placed horizontally, not just stacked up, or if arranged three-dimensionally, the hollow voids connect like a cave.
The more pieces are added, the more emptiness is created.