Our paper "Physical and geometric constraints shape the labyrinth-like nasal cavity" has been published in PNAS. In this work, we ask why nasal cavities have such complex labyrinth-like shapes like the ones displayed on the right. Our hypothesis is that this form is governed by the function of the nasal cavity. One important function is the heating and humidification of the inhaled air, which works best in narrow geometries. However, such geometries inhibit the airflow, so that breathing becomes difficult. Solving the associated fluid dynamics problem, we show that these opposing requirements can be satisfied best by a narrow geometry whose walls are separated by the same distance everywhere. In contrast, the overall shape of the nasal cavity is less important and could thus develop the observed labyrinth-like structure to obey geometrical constraints. Our theory predicts measured wall separations and shows that human nasal cavities are much smaller than expected for their body weight, which might explain why we need to breathe through our mouth during heavy exercise.