The head nod is the unwritten form of communication between males of the human species. The head nod is usually used as a greeting in passing, and can be understood as:

"Hey, howsit goin'... I want to acknowlegde your existance, but I don't want to take the time or effort to speak at the moment."

However, it is a very versatile gesture and can also take on many other meanings, such as the following:

• At a party: "Hey there! I see you have a beer. I have a beer too."
• When introduced: "Hey, yes, I see you there with my ex-girlfriend.
• Passing a cop: "Hey officer, not doing anything wrong here... just a law-abiding citizen"
• Entering a bathroom: "Hopefully my nod to you will indicate that I don't feel like talking while taking a piss.

Certainly others can come up with and add more meanings for this great symbol of american laziness.