The chain that connects the flapper in your toilet tank to the flush handle has to be long enough to allow the flapper to fall freely when the tank empties, or the tank will leak. That can waste hundreds of gallons of water every month. You can fix or prevent the problem by adjusting the chain to the right length. The adjustment is a simple matter unless the chain itself happens to be too short. If you don't have time to go out and buy a new one, a simple trick will fix the problem until you do.
Lift the lid of the toilet tank. You don't need to empty the tank for this repair.
Unhook the chain that connects the flapper to the long metal or plastic flush handle from the handle.
Hook a paper clip to the handle. If you want this repair to last for a while, use a plastic-coated paper clip; it won't corrode as quickly.
Hook the chain to the other end of the paper clip. Pass the end of the clip through a link in the chain that allows the chain to sag slightly.
Push down on the flush handle and verify that the chain is tight enough to lift the flapper high enough for it to remain open while the toilet flushes. If the flapper doesn't rise all the way, shorten the chain by a link or two.
Watch the flapper fall after the flush is complete, and verify that it completely seals the siphon hole. If it doesn't, lengthen the chain by a link or two.