It must be all that crime fiction I'm reading recently, because I had a dream where I solved a murder mystery. The setting was victorian, dark, almost gothic, and the murderer was a respectable doctor type person, who had bioengineered a mint plant to possess a thin poisonous spike. The spike was sensitive to people's presence, and 'stung' anyone who tried to pick mint leaves. So the doctor-figure only had to ask the victim to get some mint from the garden, and voila: a murder. Can't remember all the details, but it was quite entertaining.