After a two hour drive to New York yesterday with my cousin, he told me about Sweet and Sour, a game where you have to wave to random people driving their car (preferably on the highway). If they wave back, they're sweet. If they look at you, but don't wave back, they're sour.

After trying the game, the results were:

Out of 25 people who noticed...
11 waved back
5 looked and smiled, but didn't wave
4 looked and gave me a dirty look

Other ways of playing the game include giving the finger instead of waving. Enjoy!