Save for the bonobos that suck on each other’s tongues for up to ten minutes at a time, there aren’t any animals that kiss. And are we really going to count a tongue-suck as a kiss anyway? Somehow, humans are actually the only species to kiss on the mouth, and the meanings of a kiss are plentiful. A parent might kiss a child goodbye upon dropping him off at school. A kiss might occur at the end of a date as either a polite “thank you, but I won’t be seeing you again” or as an “I like you. Let’s keep this up.” A kiss on the cheek is often a polite greeting. Kisses in longer relationships are a form of maintenance, and a particularly upset person might even demand a kiss upon his derrière.