That's because in ancient times, due to the lack of knowledge, people often deified the moon and made different fantasies of various shapes. The moon bends like a sickle between the crescent moon and the half moon, so elders often tell children not to move their fingers, lest the moon representing the sickle cut your ear in the middle of your sleep.
With the knowledge of modern people, this is a superficial fallacy.
But in a positive sense, it is still useful for us to teach our children to respect the moon, that is, to respect the gods of heaven and earth and the stars of nature.