After he was captured by pirates, Diogenes of Sinope was sold as a slave to Xeniades, who had the Cynic philosopher educate his sons. “At home he taught them to attend to their own needs,” writes Diogenes Laërtius in his Lives of the Eminent Philosophers, “to live on plain food and water, to wear their hair short and unadorned, to go barefoot and without a tunic, and to be silent and keep their eyes lowered when walking in the streets.”

Fairy wren nestlings learn “passwords,” or unique single notes, from their mothers while still in their eggs; after birth they must use the passwords when calling for food, or the mothers will abandon the nest. In a 2012 study, scientists in Australia experimented with switching eggs and mothers, and found that passwords were not genetically inherited; the chicks assumed the passwords of their adopted mothers.