From Peter J comes this story about “reverse graffiti”:

From a distance, an ad on a San Francisco sidewalk might look like paint or stickers, but a closer look reveals it is actually made of nothing.

“We put down a stencil in a dirty spot, we pressure wash the lettering and lift it up, and there’s the clean advertising,” said Jason Roberts, a power washing contractor.

Pretty cool! Here’s a screengrab from the video so you can see what it looks like: