"Your sister's expecting a child," Sherlock declaimed, stalking out of the bathroom.
"My sister had an hysterectomy when she was thirty-one. Pelvic inflammatory disease," I answered, without looking up from the paper.
"We haven't had any guests in the flat but your sister and my brother since the holidays. Mrs. Hudson might have been in, but she's far too old."
At this, I looked up. He was waving a little white plastic stick with a blue plus sign showing on one end. I blinked at him and ducked my head back down behind the newspaper.
"Clara," he said.
"Hasn't been in the flat," I countered.
"And she wouldn't get herself inseminated while things are still so on-again-off-again with Harry. She's too responsible for that."
I ventured another peek up above the edge of the Times. Sherlock was tapping the pregnancy test stick against his chin while he thought. I hid my face again and stifled my laughter.
Lestrade came in. He took one look at Sherlock and turned pale. "Good God," he said.
"Ah!" Sherlock exclaimed. "These facts admit of only one interpretation!"
Lestrade groaned, "Mycroft will never forgive me, I'm meant to be keeping you out of trouble! You've impregnated some unfortunate woman, and we'll have another mad generation of Holmeses!"
Simultaneously, Sherlock (who I had repeatedly caught reading the sillier kind of internet fanfic during his long hours of not getting a good night's rest) exclaimed, "Lestrade, you bounder, you've impregnated my brother! You'll make an honest man of him, or I'll be forced to take my riding crop to you!"
Then they looked at each other, horrified, and both said, "What?"
I could no longer restrain my hoots of laughter. "Or, of course, your flatmate, the one who actually works in the field of public health, brought it home and left it in the loo to drive you spare. For the lulz."
Sherlock and Lestrade both looked put out, and I fell on the floor laughing. Sometimes the non-brilliant bloke has to make his own fun.