Do you have a toilet that fills every few minutes even when it has not been used? This is likely due to a bad flapper, flappers tend to go bad with age. But the process can happen much more quickly if the toilet has been left empty for a long period of time or if corrosive chemicals or tablets have been added to the tank.
The easiest way to test for a bad flapper is to drop some dye tablets into the tank and then check to see if the dye moves into the bowl. If this happens you’ll want to replace the flapper. If you are unable to find die tablets at a local plumbing or hardware store you can use food coloring as an alternative.