Photons are a little boring: they all move at one speed and do not interact with one another. I will present an unusual optical medium that is nonlinear at the quantum scale: I this medium, photons travel slowly, acquire mass, and exhibit mutual attraction, so strong that two photons can even form a two-body bound state. I will also present an all-optical transistor where a single photon switches on and off another light beam containing hundreds of photons.