i highly recommend doing these. haven't done one in a long while. but basically you do a shape, and find an image within it
making of video here[link]
also to preempt others who might note that i ultimately go out of the form.
that my friends is the whole essence of creativity. consider the silhouette as a summation of all the rules and knowledges given to you on how to draw. you show that you know how to work within those.
everything else. is creativity. also known as conscious choice on when and why to bend and even break those rules
also known as... and that is why i drew a horn XD