"Further" or "Farther" - What's the difference?

What is the difference between "farther" and "further"?

If you don't know which one to use, just say "further".

"Farther" is only used with distance.

"Further" can be used to ask for or give additional information about something. For example, you could ask for "further information".

If this is confusing, the good news is that "further" can be used in any case.

farther or further?