Nancy Drew is a sleuth.
After Mr. Burns was shot Lisa told her family that Nancy Drew had said that all you need to solve a mystery is an inquisitive temperament and two good friends.
When Lisa checked out Mr. Burns' library she found he had many Nancy Drew books, including the controversial Clue in the Clock. Lisa started to read the book and mused many swears to herself.
When Lisa was trying to investigate the secret classroom in Springfield Elementary School, she asked herself "what would Nancy Drew do?" Nancy Drew then said that she'd ask the Hardy Boys who she then realized are dead. Later, Lisa said "what do you think of that, Nancy Drew?" only to imagine Nancy Drew's also dead in a grave next to the Hardy Boys and Encyclopedia Brown.
When Lisa tried to find out why there was no snow in Springfield she said that if her extensive reading of Nancy Drew mysteries had taught her anything, it's that the culprit is typically the one who most benefits from the current situation.