An optical laser works by bouncing photons back and forth between two mirrors, one totally reflective and one partially reflective. If the lasing material (usually a crystal) has the right properties, photons scattering off the atoms in the crystal will excite cascades of more photons, all in lockstep. Eventually the beam will escape through the partially-reflective mirror. One kind of sorcerer's apprentice mode involving bounce messages can produce closely analogous results, with a cascade of messages escaping to flood nearby systems. By mid-1993 there had been at least two publicized incidents of this kind.