Check and Adjust Flapper and Chain

Lift the toilet tank lid. The most common cause is the flapper (the rubber seal at the bottom of the tank) not sealing properly or the chain connecting the flush lever to the flapper being too long (getting caught under the flapper) or too short (pulling the flapper up slightly). Check if the flapper is aligned correctly over the drain hole and if it's worn or warped. Adjust the chain length so there's only slight slack when the flapper is closed. If the flapper looks damaged, replace it.