Atoms in a molecule are held together by covalent chemical bonds that rely on the interaction of the outermost, or valence, electrons of each atom. Even though one refers to electrons, one should think rather of the electron density distribution, otherwise known as the electronic orbitals. The overlapping of atomic electronic orbitals determines the chemical, thermal, and mechanical stability of a covalent bond. The mechanical stability of a molecule represents a very fundamental question in chemistry. Thanks to recent advances in nanomanipulation techniques, it is now possible to address the mechanical strength of an individual covalent bond.