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Kiss from WikiPedia:

A kiss is the touching of one person's lips to another place, which is used as an expression of affection, respect, greeting, farewell, good luck, romantic affection or sexual desire. The word comes from Old English cyssan "to kiss", in turn from coss "a kiss".

Anthropologists have not reached a conclusion as to whether kissing is learned or a behavior from instinct. It may be related to grooming behavior also seen between other animals, or arising as a result of mothers premasticating food for their children.

Kissing allows prospective mates to taste and smell each other's pheromones for biological compatibility. Women are subconsciously more attracted to men whose major histocompatibility complex portion of their genome is different from their own, leading to offspring with resistance to a greater number of diseases due to heterosis, and thus having a better chance of survival.

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