Some fruit flies have evolved giant sperm, which goes against what we typically expect about sperm size. Scientists discovered that high-quality males produce these large sperm, and female fruit flies play a role by having large storage places for them. This unusual sperm size is linked to the mating behavior of the flies. The research found that the size of the sperm and the female storage organ evolve together in a way that looks like a runaway process. This study adds to our knowledge of why some animals have exaggerated sexual traits and sheds light on how sexual selection shapes mating habits and differences between males and females.