Cut your hair on a waxing moon and it grows back faster. Cut it on a waning one and it grows back slower. Almanacs still print the dates.

The belief is alive across Latin America, southern Europe and rural North America, and some salons genuinely book around it. Hair grows about 1.25 cm a month regardless of the sky. It survives partly because it is unfalsifiable at the scale anyone bothers to check. Nobody is standing there with a ruler four weeks later.