I can't find it but there is a sutta where the Buddha says that he can't think of a simile to describe how fast the mind changes, or can't think of anything that changes more quickly. Something like that. Anyone else know the one I mean?
"If beings knew, as I know, the results of giving & sharing, they would not eat without having given, nor would the stain of miserliness overcome their minds. Even if it were their last bite, their last mouthful, they would not eat without having shared."