a rolling stone gathers no moss, piedra que rueda no cría moho
-Planta quae saepius transfertus non coalescit (latin proverb)
-Erasmus's proverb gave the name "rolling stone" to people who are agile (mobile) and never get rusty due to constant motion.
Because it is so well known, this saying is one of the most common proverbs used in psychological tests for mental illness, particularly schizophrenia, to look for difficulty with abstraction. This method was used in the film, One Flew Over the Cuckoo's Nest, in order to interrogate McMurphy and to test his mental state. His answer was the following: "It's hard for something to grow on something that's moving."