There's so much to be learned from doing a white-on-white drawing on charcoal-toned paper. This exercise really trains the eye to look carefully for differences in value. It's those subtle transitions from dark to light, and vice-versa, that create three-dimensionality in a drawing.
Looking for the shapes of shadows, not just amorphous shaded "areas" will create credible volume in the objects you draw. The sensitive drawing above was done by Kristhy, a ninth grader. She successfully established a consistent light source, coming from the right, and carefully analyzed the shapes of cast shadows on the tabletop. Notice especially how Kristhy looked for the shapes of shadows on the objects.
Kristhy worked on this excellent drawing for several hours - time well spent in training her eye to observe carefully.