Presolar grains are micron-sized dust particles recovered from primitive meteorites, and the wildly anomalous isotopic signatures of these grains show that they are stellar condensates. While most presolar grains studied to date formed in outflows from low-mass stars, low-density graphite grains appear to be condensates from explosions of massive stars. Recent isotopic observations of several such grains show correlated hot spots of oxygen-18 and nitrogen-15, which indicates that the helium-rich shell in the massive star provides important source material for low-density graphites. Interestingly, much of the nitrogen-15 is produced in explosive helium burning induced by supernova shock passage. I will review the production of nitrogen-15 in explosive helium burning and delineate the key reaction pathways, including the key role of neutron-capture reactions on flourine-18. I will then speculate on what the grains may be telling us about mixing within expanding supernova ejecta.