Following the latest spate of submissions, this is another that was buried in a folder long since forgotten about. This design was printed up on a vinyl sticker and stuck to the passenger window of my car. Sometimes the passenger would ask, but often times they wouldn't. Here's how the conversation would go if they noticed...

"Um, I couldn't help but notice..." As they carefully sat in the seat, uncharacteristically slower then would be considered normal, obviously uncertain if what they quickly read as they opened the door was a joke or not.
"Notice?" I would be going through the motions of starting the car like there's no problem.
"Uh, yeah, the sticker..." I would cut them off before they finished "Oh yeah, that. Sure, there's a block of C-4 right under the center console, don't worry, it's not armed." And with that I would thump it soundly with my first. At which point they would either jump of get really alarmed, but unable to do much because at this point I'm driving away.

Tho with the recent spate of people being seriously maimed with IED's, I've decided the joke is in bad taste, so I've not done it since.