Boulder is a rock that sits upright on a cushion. Walking around the object, one notices that the object is not a boulder but only its visible surface. Given that the eye can only perceive surfaces, when observed from the front, the viewer is confronted with the optical illusion of a solid object. On the interior, a thin, grid-like steel framework supports the exterior shell. The size of the opening on the back of the rock corresponds to the dimensions of the head of the CNC mill used to carve away the stone, leaving only its thin outer sheath.