Shape and form are a good combination. Too illustrate this use a flat white
circle cut out of a piece of paper and a white styrofoam ball. Glue the paper
circle onto a dark paper. Shine a lamp in a dark room on the white paper and
explain that this is a shape, flat and 2 dimensional, next shine the light on
the styrofoam ball with a black background and say this is a form.
Explain how the form is a three-dimensional shape and show how high-light,
shade, shadow,reflected light, and backshading create the form. Now, have
them shade some forms using the 5 principles of shading, using gradations, and
they can finish in one period. Have some other styrofoam forms to see, such
as cones, cubes(board), pyramids (board), cylinders, and prisms.