An exercise i found online to create a better understanding of how a lightsource can effect a 3d object.
done with a mechanical pencil and eraser.

Basically draw your objects cubes spheres ext. ext.
then determine where your light source will be. on the opposite side create your shadow.
use the eraser after done shading to bring out where the light hits an presto 3d magic.
I apologize for it being very pix-elated. camera is cheap lol.