Greg tried his best glower, but Sherlock and John were too giggly to notice. The rest of the Yard were staring, craning their necks to see without actually getting out of their desks, but nobody except Greg knew what they were laughing at.

"And then she-" - Sherlock dragged in a wheezing breath - "-she asked John if he knew!"

"Exactly!" John was nearly falling out of his chair laughing so hard. "She - and the tulips!"

"Oh, god, yes!" Sherlock tried to grin at Greg, but he was doubled over with giggles and could barely look up high enough to see Greg's face. "Her sister was the attempted murder of course, but the tulips-"

"-were already dead!" John grabbed Sherlock's arm for balance, which had the effect of nearly toppling them both.

Greg sighed. "And this pertains to the case how?"

"Because," Sherlock panted, "the sister mixed up ammonium nitrite with ammonium nitrate! It was obvious!"

Of course. "It was?"

Sherlock sighed with clear annoyance, which was impressive considering how hard he was giggling. "Ammonium nitrate-with-an-A is in fertilizer. Nitrite-with-an-i is in rat poison."

"She-" John dragged himself back up to a sitting position. "She fertilized her sister!"

He and Sherlock looked at each other and immediately collapsed into laughter again.

Greg shook his head and chuckled reluctantly. I'm gonna kill them. Both.