Why does my toilet periodically run and how can I fix it?

Your toilet may be running periodically due to a faulty flapper valve or a leak in the fill valve. To fix it, you can try adjusting the chain on the flapper valve or replacing the flapper valve altogether. You can also check for leaks in the fill valve and replace it if necessary. If these steps do not resolve the issue, it may be best to consult a plumber for further assistance.