Philematology is the art or science of kissing.
The origin of the word kiss comes from Old English cyssan ("to kiss"), which transformed into the Middle English kissen before becoming the word as we know it today.
Anthropologists think that kissing evolved from grooming behavior or as a result of mothers premasticating (chewing) food for their children. Others think that kissing allowed prospective mates to sniff and taste each other's pheromones for biological compatibility.
To avoid clashing their noses, couples turn their faces slightly to one side when kissing. In 2003, Onur Güntürkün observed that most couples turn their head to the right - by a ratio of 2:1 - when kissing in public (like while bidding goodbyes at airports). He noted that it's similar to a baby's preference for turning the head to the right during the final weeks of gestation and for the first few months after birth.
The human mouth is full of bacteria. When you kiss someone, you exchange anywhere between 10 million and 1 billion bacteria.