Squiggles and Faces
I play this game with myself. Without looking, I make a squiggle shape on the paper, then try and turn it into a face. Sometimes I fail miserably, other times it works. Rather like finding shapes in clouds, I'll stare at a shape until it tells me where to add my own marks.
As an exercise, it proves that traditional proportions aren't necessarily essential for a characterful face.
When creating the squiggles, it's important not to make a shape that is too 'facey' so it's more of a challenge.
Here are a few examples of befores and afters.
You should try some for yourself.