I think that Grammar Girl can read my mind.  This week she (or rather her surrogate, Legal Lad) addresses the distinction between “further” and “farther.”  “The quick and dirty tip here is that you use farther to talk about physical distance and further to talk about metaphorical, or figurative, distance. It's easy to remember because farther has the word far in it, and far obviously relates to physical distance.” But listen/read Grammar Girl here and get more tips about how to distinguish these terms. Then you can impress your friends and colleagues when you correct them.  (If you are really tempted to correct someone's grammar, please listen to Grammar Girl's tips here.)