I really need to keep a scrap book. I mean, I stick things in my notebooks that I find interesting but what I really need is a systematic scrap booking system for all the bits and piece of interesting ephemera that I see, enjoy and promptly forget. The other I saw a picture of a little brass figurine with one of those rotating happy/sad optical illusion faces on the front. He had his hands out in front of him palm up and when you pressed down on the palms his face rotated. And for the life of me I can not remember where I saw it! I've been thinking about how it would work in paper on and off for a couple of days. There are a couple of interesting problems that might make for a fun project. So here's my first draft:

The face (which I drafted in Illustrator) is fixed to an axle centred between the eyes. Press one arm down and the head rotates 180° one way, press the other and it rotates back again.

The two main problems as I see them:

The head need to rotate exactly 180° them stop. Shouldn't be two hard with a suitably placed stop but I do need to make sure that is stays in place and doesn't 'bounce' back.

Secondly, the arms move only a short distance yet the head has to turn a full 180°. If I was using brass and steel I'd probably use a chain and sprocket.

This is where the interesting mechanism comes in. There are a few different things I could try. My first draft has the arms as a single unit pivoting about their middle. Connected to the arms and running over the head axle is a thin paper belt. As the arms move the belt turns the axle, rotating the face. I'm in the process of putting this together so I'll get back to you on how well it works.