Sperm swim, lung cells sweep mucus away, and the cells in the female Fallopian tube move eggs from the ovary to the uterus. Underlying these phenomena are flagella – slender, hair-like structures extending from the surface of the cells, that bend, beat or wave rhythmically. Biologists have now dissected how sperm cells of the fruit fly build their flagella. These findings pave the way to further understand the molecules and processes that may trigger a variety of human diseases and disorders, including infertility, respiratory problems and hydrocephaly, known to be associated with defects in flagellar movements.