A blank canvas can make it hard to begin. I wanted to explore how little guidance it takes to get someone drawing: just two lines suggesting a nose and a mouth, with the rest of the face left to invent.
I tried leaving the canvas empty, adding eyes, and providing a whole face. I kept coming back to the nose and mouth. They already looked like someone, even before I added anything.
Draw, next, repeat
The app opens straight onto the canvas. Draw a face, tap next, and start again with the same two lines. After a series of drawings, an overview brings them together. I wanted this short loop to make it easy to keep trying, then look back at the variations before starting another round.
One starting point, many faces
I used the same starting point to try hats, hairstyles, beards, and portraits. Pressure changes the line weight; two fingers create filled shapes.
Filled shapes 
Different hairstyles, using thin strokes and filled shapes. 
Blocks and swirls 
Trying to draw Trump from the same two lines.
Next
There’s more to figure out. Nothing about two lines tells you that drawing with two fingers creates a filled shape. I’d try a short demonstration on the canvas, then let people try it themselves. The nose and mouth also steer everything towards faces. I’d like to try other small starting marks and see what people draw from them.
I’d also put the app in front of people without explaining it first: where do they hesitate, do they find the next canvas, and does the overview make them want to start another round? That would tell me what to change in the drawing loop.