The star visited the new Tonight Show host Thursday and showed him how "to plump" his butt, a body part that O'Brien admits isn't his best attribute. (Watch video above.)
"I have no butt," O'Brien says. "There's just nothing. My back goes down, there's just leg. No butt."
Before instructing him through a squat routine, Paltrow -- who consulted her trainer, Tracy Anderson, for tips -- jokes: "You could sit in your chair and clench your butt cheeks."
To test the results, Paltrow says, "let's feel," and gives his butt a grab.