The Ivory Pill

Loot and Lilly were busy indoors with the building of a bookshelf that was to be varnished but not painted. Outdoors was an expansive hill on top of a mountain, over which kids ran, and on which nice people sat.

"Is the shelf centred?" asked Loot.
There was a pause.
"Yeah, it's very shelfish," said Lilly.
There was a pause.
"What?"
"It's very shelfish."
"Yes, I heard you, but what are you talking about?"
"It's a pun."
"On what?"
"On selfish. You said it was shelf-centred."
"No, I said, 'Is the shelf centred'."
"Yeah, but it still works."
"No it doesn't. It would only work if I said, 'Is the shelf shelf-centred' or, 'Is the shelf self-centred'."
"It doesn't matter. It's a pun."
"A pun has to work on both levels. You can't say something like, 'Is the self-centred', can you? It doesn't make sense. A true pun should make grammatical sense in both contexts. Like um... um... Like a sailor guy saying that he likes to throw little buoys into the ocean."
"That was terrible."
"It was just an example."
"Well, I still think my one was a good pun. There's no rule saying it has to work in both contexts. Go get a dictionary."
"Oh yeah? Well would you jump off a bridge if they told you too?"
"Huh?"
"I want to go outside."
"We have to finish the bookshelf first."
"No we don't. Let's get the kite."
"But... Oh... All right. Let's."