Shadows.

We have lots of these wooden figures of animals and various characters from nursery rhymes. My Grandmother used to design them and then they were cut out of wood and sold as toys. When she was designing them, she told us, one of the tests was to look at the shadow cast by a candle and see how good the outline was. I must admit I found it surprisingly difficult to get the position of the candle just about right, I think it's still not quite there but it has been an interesting exercise.