"I Say, You Say Louder" is a game.
The objective of the game is to be the one that yells the picked word or phrase the loudest. The first person whispers, then the next person whispers louder, and the next person says it, until it progresses into an intense screaming match.
Phrases often used for this game are "unicorns", "I do believe in fairies", and "penis."
This game is well demonstrated in the movie "(500) Days of Summer", where a couple is playing the game while using the phrase "penis."
Two people in a game of "I Say, You Say Louder."
1st person: *Whispers,* "Unicorns."
2nd person: *says*, "Unicorns."
1st person: *says louder*, "Unicorns."
2nd person: *screams*, "UNICORNS!!"