Every encounter begins with a greeting. Air-kissing, handshakes, high-fives, nose-rubs, cheek-sniffing and foot-kissing, sticking out tongues, floor-spitting, applause, and face-slapping--different cultures have developed innumerable ways of showing pleasure at someone's arrival. Humans have been greeting each other for thousands of years. It should be the most straightforward thing in the world, but this seemingly simple act is fraught with complications,...