Middle Son has a weird fixation (actually, he has LOTS of weird fixations, but I'm going to write about one in particular) with toilet tanks. He loves to shred paper or grass into the tank of the toilet. I've had to replace two toilets in the past three years because of this. The channels inside the porcelain get clogged and you can't clear them.
Grass is the killer. Toilet paper sometimes causes a problem, and other times it doesn't. The first time he did it he dumped handfuls of shredded grass into the toilets of both bathrooms. By the end of the weekend, I couldn't flush either one without a bucket. They both had to be replaced. My landlord made me pay for it. I couldn't argue.
One I had the new toilets, I needed to keep Middle out of them. I can't afford to replace the toilets every two weeks. My first attempt at keeping him was tying rope around the tank. That was an abject failure, couldn't get it tight enough. Within 30 seconds he had a big enough gap that he was able to slip some toilet paper inside. The next was duct tape. Many turns around the lid, then a couple around the tank. That worked.
I followed him into the bathroom. He looked at it, realized that he wouldn't be able to get it open, and he face sank. He looked like someone had just run over his dog (or rather, like someone who really cares about their dog would look if their dog was run over).
My current house, which I moved into a little over a year ago, has two bathrooms, a master bath en suite and the main bathroom. The toilet in the main bath has a cargo strap wrapped around the tank so that he can't get the lid off. That's the third method I've used to keep him out. It's worked, although occasionally I have to tighten it up. It looks a little classier (or maybe a little less white-trash) than the duct tape. I found a strap that matched the colors in the shower curtain.
Since the master bath is off my bedroom, I decided not to put a strap on it. I keep the door to my room locked, so he can't get in. Most of the time this works. He's gotten in a couple of times and put some toilet paper in the tank. It doesn't flush quite as well as it used to, but it still works.
Tonight, I caught Middle at the door to my room. It wasn't locked. When he saw me he quickly stepped back. I thought that I'd caught him on his way in. Later in the evening, I went into my bathroom, and discovered that I was wrong. I'd caught him on the way out. There was what looked like an entire roll of toilet paper in the tank. So far the toilet still works.