What's that game where you write something on a post it and stick it on someone's forehead?

I saw a video clip of the artist David Archuleta singing the song Crush and in that video there was a bunch of teenagers and they were playing a game, I think. They wrote something on a post it and stick it on the forehead of a person then later that person removes the post it and he/she reads it.
What is the name of the game? And how do you actually have to play it?