Mystery Son 

A game in which three males ejaculate into separate cups, and three females have to randomly choose a cup and turkey-baster it into their vagina. 9 months later, the females have to guess which male spawned their newborn child.

"Hey Kerri! How about you and Jasmine and Bella join John and Kyle and I for a game of Mystery Son this afternoon!"
"Sounds like a plan! Hope I get you!"

Mystery Son by FanOfGenitals November 5, 2012

mystery song 

Yet another fad from the infamous ytmnd, the mystery song was played in the background of an entry called "Demon Exterminator". It became known as the mystery song because no one could figure out what it was called or who it was by. It became an internet fad to the point that CNN even did a peice on its popularity. Its title and artist were finally discovered by the ytmnd user PocoLoco to be "Answers" by the group Vigilante.

A: Hey, have you heard what the mystery song is called?
B: STFU n00b this news is weeks old.

mystery song by Andrew Herbert May 30, 2006