Tag, a simple game often played with friends. All but one player running away from a single isolated player. Why was this player singled out? For most cases, just for the sake of the game. The player is simply handed the distinction of "It" and when the player touches another that distinction of "It" travels to the other player. Now that player is "It" or perhaps they have "It". Whatever "It" is apparently is something that every other player lacks. The game ends... whenever the players decides it ends, when the mood has ended and they no longer feel like chasing each other around, and whatever "It" is ceases to be.
However what if the game of tag had more to It? What if "It" was more than simply "It"? A word or category simply made up? A few boys playing with each other and a girl wants to join, and the boys say she's "It"? What if "It" bonded to something the girl had that the boys lacked? Then when she inevitably touched one of the other boys "It" and whatever it bonded to transfers over to the boy? Well obviously in this circumstances the girl's family's high class social status transfers over and now that the kids understand what "It" is the game of tag seemingly reverses.
That is simply It for the pitch of this story that will be written at some point. Some people decide to play tag and the person who is "It" has something the rest do not, and as such "It" bonds to that thing. So when the player who is "It" tags someone whatever "It" bonded to transfers with "It". The real questions of the story is "Who are playing?", "When does the game end?", "Who is it?", and "What is "It"?"?