REGENERCYCLING
"Doctor, what happens to the garbage we produce in the TARDIS?" asked Nyssa.
"Guess!"
"It's ejected into space?"
"Good heavens, no! That's unthinkable!"
"Burned up as fuel?"
"Try again."
"I can't imagine!"
I relented: "It regenerates."
"That's ridiculous!"
"No, I'm serious! The TARDIS converts it into raw materials and builds new rooms."
"Oh, you mean it's recycled."
"Same thing. Theoretically, we could get ourselves another Zero Room."
"Why haven't we, then?"
"Well," I mused, "theory and practice are two different ballgames. The TARDIS decides. I can ask pretty please, but…."
"So she's in charge!"
"Yes, Nyssa. She always has been."