A molecular graph is usually drawn from a set of ad hoc rules that started with G. N. Lewis. In these recipes, electron counting, chemical intuition, and interatomic distances play a major role. To answer the question, are these two atoms bonded or not? modern quantum mechanical methods use topological methods like the quantum theory of atoms in molecules (QTAIM). An exam of the electron density of a molecular system provides an unique graph which coincides, most times, with chemical intuition. The student will learn the basics of the theory. After getting familiar with it, a number of simple molecules will be computed and analyzed through topological eyes.