“Take a penny, leave a penny” (sometimes “Give a penny, take a penny”, penny tray, or penny pool) refers to a type of tray, dish or cup meant for convenience in cash transactions. They are often found in the United States in gas stations, convenience stores, and other small stores and were similarly common in Canada before the penny went out of circulation in 2013.
https://en.wikipedia.org/wiki/Take_a_penny,_leave_a_penny
How small things can add up to big change with Why Mindfulness?