Since my husband doesn't know anything about the water closet's waterworks, I went to see what was going on and indeed the water was merrily gushing through constantly. In the U.S., you deal with this problem by removing the tank lid and checking out what all the doo-dads (that's a technical term, don't throw it around or people will think you're pretentious) are up to. As is so often the case in Japan, it isn't quite as easy as all that.
Note that there is no handle to turn off the water supply attached to that pipe sprouting out the side. There's also no main water shut-off accessible to tenants of our apartment building.
Some Japanese toilets have a little pipe sprouting out of the back of the tank which resembles a water faucet only without handles. The water that is used to fill the tank spurts through the pipe into a basin with a small hole in the tank lid. This appears to be an ingenious way of squeezing in a place to wash your hands after using the facilities into an area where there is barely enough space for the toilet itself.
The problem with this feat of engineering is two-fold when you need to tinker with the tank. The lid is very heavy and unwieldy. It's got this pipe poking out of it so it's almost impossible to take it off and put it on the narrow free floor space around the toilet so you have to hold it up with your left hand while you inspect with your right.
What's worse, as I discovered last night, is that, when the water is running through and you take off the lid, the way in which this little "feature" works is for a pipe in the back that is directed straight up to feed into the tank lid and through the pipe. This causes water to literally gush up like a demented water fountain when the lid is taken off. If it went straight up (or to the right) and water fell back into the tank, that would be okay but it doesn't, it shoots up and sprays the back wall of the toilet constantly and rapidly floods the floor of this tiny window-less space.
So, just after midnight last night, I'm tired and in the toilet holding the tank lid in one hand and the ball in the tank with the other peering into the back of the tank to figure out how to fix it. I see that the problem is that one of those disks that I put in the back of the toilet to automatically clean it has dissolved into a pile of mush and is clogging the stopper in the back. I have to let go of the ball, grab the gob of goo and throw it out but that means letting go of the ball which causes the water to spurt through the pipe with the force of Niagra Falls and hit the lid in my left hand and run down the side of the tank onto the floor.
I really have no choice so I do this as fast as I can and rush back in and try to get the lid back in place. This is where the fun really starts. Because the lid is so heavy, I can't align the little pipe leading into it with the pipe gushing water one-handed. I have to let go of the ball and allow the water to gush up or out the side while holding the lid two-handed. If at least some of the water would fall into the tank, it'd fill up, the ball would float and it'd stop but the water spurts anywhere but into the back of the tank.
It takes a half dozen attempts to get this to work and by the time I'm finished, the water closet (which is actually smaller than a real closet) is flooded. The bathroom carpet is saturated, the back of the room has a shallow pond and the back wall has gotten a royal bath, my slippers are soaked, and the arms of my long-sleeved nightgown are wet (and freezing) and my arms are smudged with the silty stuff that has collected from nearly two decades of hard water running through the tank.
I will never look at that toilet design again and marvel at what a water and space-saving idea it was to put that spout on the back.