Values are the degrees of light and darkness in a drawing. They give additional form and depth to a basic structure. Observing the wide range of values that make up your subject will give you a better understanding of how light creates highlights and shadows on the form.
Observing Values Exercise
Take white foam shapes from your local craft store, set them up with a good light source, then observe the characteristics of the shapes, the highlights and how the shadows fall. Notice the wide range of values. You may need to paint the foam white to get an opaque surface that reflects light smoothly and accurately. Be sure to use latex paint, because spray enamel will melt Styrofoam.