I had a really crappy roommate one time- absolutely no respect for boundaries. Eat all my food, leave the house trashed etc. One of his favorite activities was to pick my lock when I wasn't home and smoke my pot. Every. Damn. Day.

So one day I was cleaning before work and found some concentrated Salvia extract from my brief foray into ethnobotanicals. Salvia is a powerful and quick-acting (legal) plant which induces full-blown visual and auditory hallucinations for about 15 minutes when smoked. It's not exactly unpleasant, but I wouldn't call it fun either. Naturally, I pack my bong full of the stuff and leave it in my locked room.

Come home from work, the bong has been smoked. My roommate spends the rest of the night dropping hints to me trying to figure out WTF he smoked without coming out and admitting that he broke into my room and stole from me. I never said anything and he never touched my stuff again.