Abstract: A three dimensional polyhedron frame structure formed by a plurality of rigid rods and a plurality of hubs in which each rod has a retractable bolt at at least one end for assembly into a socket of a hub having a plurality of sockets that accept a plurality of rods with the rods being assembled to the hubs in different directions to form frames of various polyhedron shapes. A board is attached to at least one horizontal rod forming a vertical face of a prism frame so that its angle of inclination can be adjusted to demonstrate a principle of gravity. Preferably the rods are electrically conductive and the hubs are non-conductive so that wires can be connected to selected rods to form an electrical circuit.