The Doctor did a horrible job at hiding his pleasure and amusement from watching Jonathan's expression. He covered his mouth with his fist to hide his smirk, and he kept glancing over at Bill, mirthfully.
"Oh, this is just the Console Room. It goes further. Welcome to the TARDIS... Time And Relative Dimension In Space." He was waiting for Jonathan to say those words that everybody wound up saying it's bigger on the inside!