PHILADELPHIA (CBS) — What if you could clean up the city and give clothes to the needy? That’s the idea one girl and her father had after seeing what gets thrown away.

When marathon runners reach the finish line, they have often left clothes behind. That’s what 13-year-old Madeline Resnic discovered when she attended her first Philadelphia Marathon in 2007.

“I saw the clothes being discarded, and I didn’t understand why it was happening, because there are so many homeless people and I see them all the time,” Madeline said.

Her father, Michael Resnic, asked around.

“They explained to us that runners like to keep their muscles warm as long as possible and then discard their clothing when they start,” he said. “We ran home and grabbed a trash bag and picked up as much clothing as we could.”

That day, a big idea was born: Clothes-Pin. Pin stands for “people in need.”

Now, the Resnics attend races–not only in Philly but as far away as Washington, D.C.–with volunteers and pick up discarded clothes to donate to homeless shelters.

“The first time I did it, I thought it was so strange that these clothes that looked identical to something like this were just being thrown away,” said Madeline, pointing to a stack of nearly-new shirts.

They have collected an estimated 50,000 pieces of clothing and outfitted countless people they’ll never know.

“All the shelters are really grateful for all the donations, because they really need them, especially in the wintertime,” said Madeline.

It’s an idea with staying power.

For more info on the organization, click here.

Reported by Dave Huddleston, CBS3