I have a sign in the back window of my car, along the bottom edge where people like to put stickers announcing the academic success of their offspring, except that it says something like the pictured bumper sticker.
As I cruised past a chap in screensaver mode and wearing “MSCP###” on his number plate (where ### is some random number), his numbed after-work gaze travelled across to my rear window, then stopped. Presently, he frowned. Then the penny dropped and the lights came on inside. He started laughing. And laughing. And laughing. And drove across the lane markers (“brrrump, brrrump, brrrump”) a couple of times, scaring the lady behind me into driving all scribbley herself and slowing down to leave an empty spot next to him. He was still laughing when he exited.
I guess it made his day, too.