I keep a squirt bottle in class. Its primary purpose is for cleaning the overhead projector, but its secondary purpose is somewhat more, how shall I say this, disciplinary. Feet on the desk? Squirt. Sleeping in class? Squirt. Do something I think merits getting squirted? Squirt. It's most effective.
Students are forbidden to touch the squirt bottle. In fact, they're forbidden even to think about touching the squirt bottle. Why? Because I said so. You disagree? Squirt.
Someone has repeatedly broken the sacred rule. For this he/she should get beatings. Many, many, (pause and deep breath), beatings. A few times this year, someone has soiled the squirt bottle. He or she sticks an overhead pen in the bottle, turning the water to a pleasant shade of purple. It's dark enough to see the purple when looking at the bottle, but when the water is squirted (onto the overhead, of course) it appears clear. Therefore, it must be safe for, you know, other uses.
I allow students (from 4 of my 5 classes) to be in the room when I'm not there, such as during breaks between classes. That is when the culprit apparently makes his/her move. Nuh guh culprit.
There was almost a break in the case today. One student blurted out to me the name of the person doing it. Ah, success! He begged me not to let the criminal know who told him. Right before the end of class I called that student over. "You are so in trouble. Not detention/suspension/vice principal kind of trouble--oh no, it's worse than that. I'm onto you. I know who you are and what you do. You *will* pay. Watch your back. When you least expect it, expect it." Of course I said this with a somewhat dramatic voice, but the kid seemed a smidge concerned. What do you think I did, he asked? "You know what you did." Do you think I talked about you behind your back, he quivered? "You have your First Amendment right to say whatever you like about me. But you know what you did." At that point I put the squirt bottle on top of the overhead. "Do you think I colored the water?" he asked?
Now we're getting somewhere. "Did you?"
After more intense interrogation techniques, he volunteered the following information: someone from his class sneaks into the room between classes when I'm in the restroom, filthies the water, and then leaves the classroom to come back a few minutes later--to be above suspicion. Devious little devils, aren't they?
He said he'd tell me who it was. I declined his offer; I will find the conspirators myself. And when I do....(evil genius laugh)