Fix a running toilet! Mechanisms vary, but they all work on the same principles. Flush a couple of times while you watch in the tank with the tank lid off and notice the process.
When you push the handle, the chain lifts a flapper, letting a tankful of water fall through the opening in the bottom, into the bowl. As the water level drops, the flapper drops and closes the opening.
A plastic float drops as the water drains. The float is connected to a valve that lets water into the tank when the float is down and stops (or should stop) when the float is up.
In the middle, there’s also an overflow tube that drains water out into the bowl if it gets too high.
Catch it in the act. If you've waited long enough after flushing and the toilet hasn't quit running, lift the tank lid and look in.
Close the flapper. If the tank is not full and it is not filling, chances are that the flapper is stuck open. Reach in and close it with your hand. If it sticks repeatedly, look for the cause. Make any necessary adjustments. Is the chain catching on something or is the flapper catching on the chain? Try threading the flapper chain through a plastic soda straw to prevent a long chain from getting stuck on things and preventing the flapper from seating properly. Or, replace the chain completely with a loop made from dental floss that is the same length as the chain. Is the flapper wedged open on its hinge? Is the flapper aligned with the opening? If you have a ball seal instead of a flapper, is the wire that lifts the ball straight and does it move freely?
Clean or Replace the flapper &/or flush valve. If the toilet stops filling and then starts again intermittently or water constantly runs into the bowl, you have a slow leak from the tank into the bowl. Try this to be certain. Place a dye tablet or a few drops of food coloring in the tank. Your local hardware store may have free dye tablets for this purpose. If, after an hour or two without flushing, you see this dye in the bowl, you have a slow leak, a small amount of water running into the bowl. The most common cause of slow leaks is a leaky flapper. Over time, this inexpensive rubber part may decay or get old and stiff to the point that it needs replacing./) or minerals may build up on it &/or the rim of the flush valve where it seats. If the flapper is still in good shape, sometimes all it takes to make it work is to clean it &/or the rim where it seats. Run a finger carefully around the underside of the flapper & the rim where it seats. Remove any uneven buildup of minerals that might cause a leak. Use a sponge with bleach or steel wool or #500 wet-or-dry abrasive paper. Cleaning may work to remove mineral buildup, but it's usually best just to replace the whole part. There are a few standard kinds, so take your old one with you to the hardware store for comparison (to ensure you get the right kind). To perform a replacement: Close the water valve and flush the toilet. If the valve is completely closed, the tank will not refill and you will not hear water running after the tank empties. Pop the old flapper off its hinges, disconnect it from the chain, and pop the new one into place. Don't forget to open the valve all the way when you're ready for water again. Try flushing a few times to make sure the chain is the right length for the new flapper. It should open when you push the handle and then drop closed all the way when the tank empties. You may have to trim and adjust the chain by trial and error. Also, make sure that the flapper aligns properly with the opening.