A model of a cube made from a sealed envelope.

A model of a rhombohedron-necessary for teaching stereochemistry (especially of inorganic compounds)-can be made easily by using a sealed empty envelope. The steps (illustrated in the figure) are given below.1) The envelope is folded down the center lengthwise.2) The lower part of the envelope is folded up so that corner B falls on the center line. The new corner on the right-hand side of the envelope is marked C. 3) This lower part is folded further along line AB so that point C falls on the left-hand side of the envelope. That point is marked D. The new corner on the upper part of the right-hand side of the envelope is marked E. 4) The horizontal line EF, perpendicular to the right-hand side of the envelope at E, is drawn, along which the upper part of the envelope is cut off. 5) The remaining portion is flattened out and its lower part is folded up at point D so that corner A falls on point C.The new left end of the base is marked G. 6) The upper part, of the envelope is folded down at point C so that corner E falls on point D. The new upper right corner