So, for example, if we have molecule one and molecule two that both consist of these carbon atoms, so we have two carbon atoms bonded, two carbon atoms bonded at one moment in time. This could be the electron density. And because we have symmetry, we will not have a dipole moment. But at the next moment in time, the electron density of this can change to this, because the electrons are continually fluctuating over time. So if some of these electrons fluctuate to this side, that will give this carbon a partial positive charge. This carbon a partial negative charge.