To your toilet flush on its own, this is a problem known as ghost flushing. The most common culprit is just a leaky flapper that needs to be replaced. Begin by turning off the toilet's water supply. Then flush the toilet and drain the tank of water. Next, detach the flapper chain from the flush lever arm inside the toilet tank. Remove the old flapper from the flush valve. Install your new corky flapper by attaching the flapper ears to the tabs on the flush valve.