Abstract: Free radicals, characterized by their unpaired electrons and high chemical reactivity, have long been simplistically portrayed in public discourse as solely responsible for aging and disease, while their vital roles in biological processes, industrial synthesis, and ecological regulation remain underappreciated. This educational article employs a “chemical courtroom” narrative framework to present a balanced perspective on free radicals. We first elucidate their formation mechanisms, classification, and intrinsic reactivity-lifetime relationships that explain their inherent instability. The discussion then examines their detrimental effects, including DNA oxidative damage, protein carbonylation, lipid peroxidation, and material corrosion. Countering this perspective, we highlight their indispensable functions in immune response (oxidative burst), nitric oxide signaling, polymer synthesis, advanced water treatment processes, and atmospheric ozone chemistry. The article further delineates the comprehensive antioxidant defense system comprising endogenous enzymes (SOD, CAT, GPx) and exogenous compounds (vitamins, polyphenols, nanozymes), illustrating the delicate biological equilibrium maintained with free radicals. Grounded in the principle of “the dose makes the poison”, we emphasize the distinction between beneficial utilization and harmful excess. By combining engaging storytelling with scientific accuracy, this work seeks to rectify the one-dimensional negative perception of free radicals, promote context-dependent evaluation of reactive species, and provide a valuable resource for science communication and chemistry education.

Key words: Free radicals, Reactive oxygen species (ROS), Reactive nitrogen species (RNS), Oxidative stress, Antioxidant system