There is a knothole in the floor of 221b, off in the corner of the dining room. It is beneath a cabinet, so it is not readily visible and therefore has never been filled by the flat's inhabitants. The other side of the knothole is in the ceiling of Mrs. Hudson's parlour, and thus while she would never eavesdrop, when the conversation above reaches higher volumes she can usually make out its content quite clearly.
"No."
"Oh, but Watson…"
"Absolutely not. This discussion has officially ended, Holmes."
"You're the one who suggested we take a holiday!"
"A relaxing holiday! Not some crazy joyride!"
"I'd hardly call it a joyride, Watson, it's all very controlled, and it would only be one event. Think of the French countryside as seen from up in the sky!"
"Think of crashing into the French countryside in a fiery wreckage of carnage and wicker."
"You know, if I wanted someone who never tried anything new to be a constant companion, I would have simply roomed with Mycroft. Where's your sense of adventure?"
"I must have left it back in Afghanistan as I was dodging bullets and fighting for my life."
"That's a low blow and you know it."
"I'm not going ballooning with you, Holmes, and that's my final word on the matter."
"This is why we can't have nice vacations!"
People often ask Mrs. Hudson why she tolerates the worst boarders in London, and when questioned all she can do is smile and shake her head.