Franklin compares idleness with rust, which damages metal through disuse, while ordinary labour produces only gradual wear. The bright key extends the image: frequent use prevents it from becoming dull or corroded. Together, the comparisons suggest that inactivity can weaken a person more than purposeful effort does.

The saying reflects Franklin’s wider emphasis on industry and regular activity. It should not be read as an argument against necessary rest, since recovery is different from habitual idleness. His broader point is that abilities, habits and opportunities can deteriorate when they are consistently left unused. Continued engagement, by contrast, helps preserve skill, discipline and readiness. The proverb therefore presents productive activity not merely as a way to accomplish tasks, but also as a means of keeping one’s capacities active and effective.