Most parts of the real world can be described via interacting particles. Whether you look at the microscopic level and see molecules bumping together or try to understand how electrical impulses in your brain form thoughts, one question is common to all particle systems: how can local interactions induce a macroscopic behaviour? In this week's "What is... ?" seminar, we will see how particle systems model natural phenomena, where randomness comes into play and what behaviour we might expect to see at different scales (=zoom levels).