1) A Dirty Car
You do not necessarily need to own the car in question, you just need to have physical access to it. In fact, many of you may ultimately prefer to use a car that you do not own.
2) A Finger
This should probably belong to you.
Write the following on a surface of the dirty car:

The car pictured above belongs to my friend Jon. After I took the liberty of decorating it we encountered a delightful variety of reactions:
A bunch of kids walked by with their parents. The kids read the words out loud as they passed by and said "I don't get it." Their parents, on the other hand, almost fell over laughing.
We walked out of a grocery store to find two large bikers dressed all in studded leather and their old lady (also dressed in studded leather) laughing over the car.
Biker (pointing and laughing): "That's frickin' hilarious, man."
Jon (scrambling for his keys): "Umm. Yes. Thank you."
The next day at work Jon even found a response written underneath:
"She is with me."
Sadly, the car received a long overdue washing before this brilliant rejoinder could be immortalized on film.