I run life drawing classes in a hall near where I live, and part of the teaching involves doing small quick drawings to illustrate certain points. Two here show how I was demonstrating to a student how light & shade can be used to give solidity to a form. If we think of the body as a series of solid shapes, we can see that the planes facing the light source will be brightly lit, while the planes that face away will be in shadow. For instance in drawing 1 only the top is lit while most of the back, from where this student was, is in shadow. In drawing 2 the light was on her front, so the sides of the body,legs, arms etc are in shadow.