A loud explosion caused John to sigh, and his younger sister to jump from where they both sat in front of the fireplace.

"What happened this time, Sherlock?"

John asked, exasperation shading his tone.

Sherlock's voice called out from the foggy cloud of bright green smoke spilling out from the kitchen.

"Just a minor setback! It seems that when you combine sulphuric acid dilute with bicarbonate of soda and the dust from under the sofa, this happens..."

He gestured to the slowly diffusing cloud and tapped his chin(s?) in thought.

"I wonder what would happen if I mixed iron filings and fire? Or run carbon dioxide through lime-water?" He mused.

Elsie sniggered from where she sat on the floor, holding her chemistry textbook aloft for her brother and his flatmate to see.

"Sherlock, we've done both of those experiments at school!"

Sherlock frowned.

"And what's more they were the introductory year seven experiments!"

John snorted loudly and gave his little sister an appreciative glance before turning back to Sherlock.

"Sherlock, you've just been outwitted by a fourteen year-old school-child!"

Sherlock pouted.

"Oh come on, Sherlock, there's no need to pout about it!" both Watson siblings chuckled in perfect sync.

Sherlock only responded by retreating back to the kitchen and proceeding to, for lack of a better word:

Sulk.