I once noticed how a newspaper would become yellow, except for square covered by a magazine. So I put newspaper with a large unprinted area in an old camera, le

I once noticed how a newspaper would become yellow, except for square covered by a magazine. So I put newspaper with a large unprinted area in an old camera, left it exposed for a week and behold, it did show a vague skylines. Later I used red post-its, since red really bleaches in sun. Also worked, though not significant better. A fun experiment.