Originally posted by: jupiter57
You are having problems with what is called a "Flapper", a black rubber gasket with a "bulb" on the bottom of it.
It's designed so that when you push the handle, it pulls up the flapper, the hollow "bulb" on it holds it up while the tank is releasing the water.
When the water level in the tank falls low enough, the flapper falls back in place.
The chain tbike speaks of comes from the handle down to the flapper.
If adjusting it doesn't help, you need a new flapper.
(Note: This Flapper is used on most modern day toilets, if it is an older toilet, it may have a variant of this, sometimes a plastic "tank" that slides up and down the standpipe)