Is there some unwritten rule I don't know about? Over the weekend I was riding around in my sister-in-law's new yellow Jeep Wrangler.

I noticed that people were waving to us. People we didn't know. People in other Jeep Wranglers.

I asked her about these strangers in similar vehicles waving. She laughed and said to wave back. I asked if this was like the motorcycle riders who do the two-finger wave that means keep the rubber side down. She said, laughing, "yes."

Then I started to wonder, do only certain cars do the wave to each other? And how do you know which cars? Should I be waving at other Ford Edges? What are these unwritten car waving rules?