The inset appears to move.

*Fruits of pine trees.

Source: A. Kitaoka

“It is better to have enough ideas for some of them to be wrong than to be always right by having no ideas at all.”
– Edward de Bono

One of my favorites. FedEx’s well-known logo has a right-pointing arrow created by the negative space between the E and x. While the arrow becomes quite obvious when pointed out, most people don’t notice it otherwise. An excellent study in subliminal advertising, with the arrow symbolizing forward movement and thinking.