See a Penny, Pick It Up, All Day Long You'll Have Good Luck
"The belief that finding and picking up a stray penny brings good luck for the rest of the day."
A relatively modern American twist on a much older superstition. The original version wasn't about pennies at all, it was "see a pin and pick it up," tied to a pagan belief that metal was a protective gift from the gods, and a stray pin might have been used in a good-luck spell. As coins became the everyday precious-metal object people actually encountered, the rhyme drifted from pins to pennies, carrying the same underlying "found metal equals luck" logic along with it.
Sources: 'See a pin' pagan good-luck tradition, documented via USC's Digital Folklore Archive and related folklore references.