chewing gum

[benji]

why does chewing gum turn white on black surfaces and black on light surfaces?

[Tiesto]

They do turn black on black surfaces once they've been on there a while, it's just that you dont notice them unless they've recently been deposited and their white and havent yet collected enough dirt to make them black.

[natalie_1982]

In the same way that if you place your hand on a black board it will look lighter than if you place it in front of a white board.  The white board will make your hand look darker.