Cosmic ray detectors such as the Pierre Auger Observatory in Argentina and KASCADE in Germany detect air showers – cascades of charged particles – that are induced by high-energy cosmic rays as they collide with nuclei in the high atmosphere. NA61 measures the production of hadrons from pion-carbon interactions – an approximation of what happens when pions interact with nuclei of air. The NA61 results will improve the understanding of the nuclear cascades in the air showers and thus help to deduce the properties of high-energy cosmic rays from air-shower data.