In the set of actions and reactions that take place in the organism, oxygen exercises the role of transforming the food we digest into energy. It is a process in which 95% of the oxygen is transformed into water and the remaining 5% reacts with the excess cholesterol, resulting in free radicals. But what is a free radical? Here, we seem to have come across the final villain on our tour, and it is worth giving greater attention to it. A free radical is a molecule or a fragment of an unstable one. Unstable because it is produced when a normal molecule - whose electrons, which unite the atoms, are associated to pairs, lose or gain an electron. In other words, the molecule loses one of the electrons and becomes unstable, as it is always seeking an electron to achieve stability. It is like a couple who lose each other. Solitary, both will end up spending the whole journey trying to steal a partner from every other couple, in such a way that every marriage will lose stability. Imagine a world where nobody knows who is who, and everybody wants somebody. This is what occurs in an organism full of free radicals. In this chain of creating and destroying pairs, the radicals cause an alteration in cell composition, which may entail damage to the cellular membranes, tissues and even portions of the cellular DNA. It is a chain reaction that can develop organic disturbances, causing degenerative diseases, premature ageing and cancer. However, it is understood that oxygen is only the transforming substance, which, in the final instance, gives form to the noxious agent that acts on the organism. Our lives, sat in front of a computer, an inadequate diet (food produced with sweeteners, flavorings, preservatives or anabolic steroids), the competitive, stressful way of life are, in fact, the main forming agents of free radicals.