This is not a tutorial for how to FIX a chronically running toilet. It IS a tutorial oh how to STOP a toilet from running once, and includes a brief explanation of how the toilet works. If your toilet is chronically running, one or more parts in the toilet will probably need to be replaced (don't worry, I just saw these at the hardware store, and they are pretty cheap). So there won't be a tutorial on that unless my toilet continues to run =).
First, take off the lid to the back of the toilet. I had never ventured into this territory until I was a missionary in a rural area in Guatemala. Since I preferred not to use the "toilets" most people had in their homes (aka hole in the ground somewhere out back), I lost any hesitation to do necessary maintenance to the toilet we were lucky enough to have in our place.
Okay, back to it. Welcome to the inside of the toilet:
Pardon the grody-ness. We just moved in and I can only do so many things at a time... So all the parts we come in contact with have been cleaned, but I haven't gotten to the tank.
Crash course on how toilets work:
When you pull on the flusher, that black lever goes up. It pulls on the chain, which pulls up the floating white/orange bob and the blue plug, which unstops the hole in the bottom of the tank, letting all the water flush down into the toilet bowl. The white/orange bob floats on the water, which keeps the plug from closing until the water level drops enough to flush the toilet.
That big black thing on the left controls the water that runs in to refill the tank after you flush. It floats, so when the water level drops, it slides down the pole it is mounted on, and that allows water to flow from your pipes into the tank to refill it. When the tank is full enough, it will float back up and stop the water from flowing into the tank.
Knowing that, a running toilet can really only be caused by a few things (knock on wood). The one I have seen most often in our places is that the plug gets caught on the lip of the hole and doesn't close all the way, so the water keeps going into the bowl and doesn't fill the tank. To fix this, either wiggle the chain to get the plug to fall, or stick your finger down and poke that thing back into place. Done.
Sometimes the chain gets twisted or tangled for some reason or another. Untangle that thing and forget about it!
Sometimes the flusher (on the outside of the tank) sticks down,
which keeps the lever on the inside up, preventing the plug from
closing. Pop that little fellow back up into place. Finito.
And sometimes that big black thing (I really should look up the proper name for it) doesn't slide all the way up to stop the water. This is more likely the case if the tank is all the way full and still running. Grab it and gently slide it up until the water stops. Golden. This is the one that happened to me today.
If the problem continues, look into repairing or replacing the culpable part.
And that, my friends, is how to stop a running toilet. [Insert joke about a toilet that is physically running here.]