Mirror, Mirror on the Wall

In 1656, Diego Velázquez painted himself painting. But he's not looking at his canvas — he's looking at you. The young princess Margarita Teresa stands center stage, flanked by her attendants (the meninas), but a mirror in the background reflects the king and queen, implying they stand where you stand now.

That mirror changes everything. It means the real subject isn't the princess — it's whoever occupies the viewer's position. Velázquez essentially built a trap for your gaze. You walked in thinking you'd observe a painting, and instead the painting started observing you.