Semen! With the possible exception of Bud Light, no fluid is as responsible for the propagation of the human species.
Some of us think it's funny, some of us are grossed out by it and the lucky few of us cannot get enough of the sticky swimmers.
Despite it's ubiquity (have you seriously ever taken a black light to a hotel room?), we're still finding out new and newer things out about the mother-of-pearl goo.