Those of you who role-play elsewhere will already have a leg up on this, but for those who don't, it's very simple. Find (Tumblr's the #1 source) and post an animated gif in response to the one the person above you posted, effectively creating the illusion that your gif has just reacted to the one above you. Clever commentary below your gif is optional, but it can be funny--and that's the point of the game. A hilarious multifandom extravaganza of epic proportions.