Building block

A building block adapted to be assembled with other such blocks to form a structural unit, each of said blocks comprising a hollow tetrahedron body having four identical side walls shaped as equilateral triangles, said side walls being interconnected along ridges which terminate at tips of substantially conical corners, each of the side walls having a central opening provided with side edges, said side edges of each opening defining an equilateral triangle and being equidistantly spaced from and disposed parallel to adjacent ridges, said corners each being enterable into an opening of an adjoining body to abut the side edges thereof, the relative sizes of the openings and the corners are such that the tip of a corner lodged in an opening of a vertically standing body is disposed at the vertical axis of said body.