Replaced another flapper

It is always handy to keep an extra toilet flapper around to deal with leaky toilets, especially if your house has 3.5 baths. They are easy to replace:

http://kevinscoffeebreak.blogspot.com/2015/09/running-toilet.html?m=1

Sometimes the flapper will just get some dirt or debre on it that will prevent it from sealing. You can try cleaning the flapper if you want, or, it is easier (and not all that expensive) to just replace it.