Oh yeah? Well here's another one for you...

Proposed:

The level of complexity and enthusiasm with which two strangers performing an activity in public acknowledge one another nonverbally is inversely proportional to the total number of people who engage in that activity.

Examples:

Two strangers pass on a crowded street in a big city. There is nothing special about this, as millions of passings are happening in that city every second, so no acknowledgment is necessary. In fact, nonverbal acknowlegement of the strangers who share the commonality of walking on a sidewalk in a big city would drive everyone insane and basically shut down society.

Two strangers pass on a sparsely populated street in a small town. They exchange a very brief, almost imperceptible nod, possibly involving a millisecond of eye contact. This is because they share the somewhat distinctive quality of being one of the few people on the street, and this mildly unusual commonality must be acknowledged.

Two strangers pass each other while riding bikes on a public path at high speed for exercise purposes. They exchange a visible nod. These individuals do not nod to any of the walkers, roller-bladers, or casual bikers out on the path, but only to the sweaty, windblown minority who exercise they way they do.

Two strangers pass each other riding motorcycles. They exchange a nod and a half wave, the arm typically coming straight out from the body rather than being extended upwards. Not a lot of people are insane enough to pilot an engine loosely associated with two wheels and a handlebar, so when these individuals pass, they must do a bit more than most people acknowledge the eliteness of their biking fraternity.

Two strangers pass each other piloting boats. They exchange the classic "boat wave." Less low-key than the motorcycle wave, the boat wave is a raised arm and hand with, depending on the dorkiness of the driver, a little bit of wrist action. Simplified, this means: "Hey, I have a boat." The reply wave, identical, means, "Hey, me too." Boat waves are more vigorous than motorcycle waves because there are more motorcycle owners than their are boat owners (source: it's late and I said so).

Two strangers pass each other driving 1973 Chevrolet CanAms. They both drop their pants, stick their bare asses out the window, and spank themselves with one hand while giving each other a rotating rock/paper/scissors gesture with the other hand. The demonstrative nature of the gesture is necessitated by the rarity of the automobile, and also possibly because of its hideous appearance.

Here endeth the lesson.