In the “Star Wars” movies, Jedi knights engage in dazzling duels with lightsabers that confine light and push it against each other. However, confining photons or enabling their interaction in reality, especially in free space, is extremely challenging. Photon-photon interactions, which are only possible through optical nonlinearity, are difficult to achieve with conventional materials. The quest to confine photons in a specific space for as long as possible, and to allow individual photons to interact with each other, is a major challenge for researchers in physics and optics. Since the invention of the laser, the study of optical nonlinearity has been the foundation of various modern scientific and technological advances that contribute significantly to our daily lives. Recently, optical nonlinearity has become a central platform for quantum information, computing, and sensing research, highlighting its growing importance. This article discusses a new turning point in optical nonlinearity based on micro-resonators, and presents efforts and future perspectives to realize photon-photon interactions.