It's Always Sunny in Philadelphia S03E01 - The Gang Finds a Dumpster Baby
Dennis, Dee and Mac find a baby in a Dumpster, and Dee and Mac decide to raise him. They also decide to use him to raise some cash. Meanwhile, Dennis joins an environmental-activist group, and isn't exactly welcomed with open arms. But he finds a way to take revenge. And Charlie wants Frank to take a DNA test to determine whether they're father and son. Frank won't do that, but he's happy to join Charlie on Dumpster-diving expeditions.