A running toilet is easy (and usually cheap) to diagnose and fix. The above suggestions are the two most likely culprits, and I would also add that if you have *very* hard or rusty water, you may need to drain the tank (turn off the water supply and flush) so that you can take a pumice stone to the area where the flap sits to enable it to seal again. A replacement flap kit is readily available in any hardware store and costs less than $10. You can do it and you will feel so proud after you do!
