After overhearing Mrs. Hudson 'entertain' a guest, John knew his responsibility. Unfortunately, he couldn't complete the task solo. So when he set the teacup at Sherlock's elbow that morning, he asked as nonchalantly as possible, "Are you available for an experiment today?"

Sherlock raised his head slowly, face carefully blank, but his eyes gleamed and his heart rate ticked up. Since he was hiding his reaction, John wondered how many people had manipulated him with word 'experiment.' John guessed that Mycroft had for decades. "Oh?"

"Mrs. Hudson needs a white noise generator. I'll need to test all of the store options."

Sherlock bounced off the couch. "Let's go buy them all and bring them back here and..."

"No. I am going to drink my tea. Then we are going to the store and try all of the options there and buy only one, the one the best counters my hearing."

"Then anyone could deduce which one works best. And there'll be distractions, really John, you know how unaccounted for variables might cloud a conclusion."

"Tough. I'm sure you know that Sentinel experiments in a sterilized lab setting are notoriously inaccurate. Drink your tea."

Sherlock sat down and consumed his tea petulantly. He packed up his laptop and waiting at the door by the time John was ready. Sentinel shopping with Sherlock was an experience. He sat in the middle of the aisle with his computer and recorded every option and its specs. John bought a package of batteries and began turning on the various options. When John refused a generator, Sherlock wanted to know Iwhy/i. Thankfully, Sherlock was enough of a distraction that John never spiked or zoned the entire trip. The only viable option was also the most expensive. John winced at the price, but Mrs. Hudson was worth it.

Only Mrs. Hudson was confused with the gift. "Did you give Sherlock one?" she asked.

John laughed. "If Sherlock wanted to hide something from me he would never do something so obvious as use a white noise generator. He knows a hundred other ways to keep a secret." Sherlock considered any knowledge that could be deduced to be public domain, including items pertaining to himself.

"I would rather you be aware of your surroundings than protect my privacy. Take it back, dear. I have no need for that thing." She patted John's cheek and shut him out.

Stunned, but John chuckled as he climbed the stairs to their apartment. The moment he opened the door, Sherlock called out, "Mrs. Hudson refused the white noise generator, of course."

"How do you know?"

"Obvious! She was thrilled when I told her that my flatmate was both a doctor and a Sentinel. She would never limit your senses. "

John walked into the living room. "If you knew she would refuse it, why would you agree to go shopping with me?"

Sherlock's look spoke volumes. "Really John? You offered to run an experiment with your senses. Naturally, I would want the data."