It's kinda twisted... but fun.
I may have mentioned that as an undergrad I lived with a cool roomie, and a moron. Well, they had gotten used to me having strange samples lying around for Micro lab (tube of river water I'd collected for lab, plates I'd swabbed from the bathroom sink, etc... all innocuous, and nothing too threatening.) Soon after I started working in one of the research labs, however, things changed a bit. See, the lab focused on replication of herpesviruses and HIV. That made the two of them a bit nervous.
One day, I decided it just had to be done. I took a microfuge tube (bout an inch long and with a snap-cap) and filled it halfway with ginger ale. Then I froze it (Looks exactly like a virus pellet--not that these guys would know, but ominous, nonetheless). I labeled it in big, red letters as HIV, with some gobbledygook official-looking stuff alongside, and then I popped the cap open and put in on the frozen peas in the freezer.
When the poor sap got home, I just calmly told him I had a sample in the freezer, and that it's all good as long as you don't open it.
Then, I waited,
A few hours later, I heard a shriek from the kitchen. He'd read the label... and was freaking out. I actually had to drink it to get him to calm down. All in all, it was a goooood day.