I like all the people who say, oh this is just Wave / Particle Duality. That's really old. It's already been explained. NO. This experiment is still amazing in the mind warping way it destroys any attempts at trying to visualize matter / energy. The photoelectric effect shows that photons act like particles, yet we saw here that they also act like waves. Even when a single particle is being shot it acts like a wave. That's messed up. And while probability is a nice, it's a tool, not an answer.