It's Always Sunny in Philadelphia Season 3 Episode 1

The Gang Finds A Dumpster Baby

Dennis joins up with an environmentalist group, but his altruistic intentions quickly sour when he's snubbed by the activists. Dee and Mac decide to raise a baby they find in the dumpster, but quickly find out that it's not nearly as fun as they thought it would be, and Frank and Charlie take the concept of recycling a bit too far.