So is it octopuses or octopi - and how do you know?
I'm often confused about how to pluralise words ending in -us, as there doesn't seem to be an easy rule. Then someone told me that words derived from greek go from -us to -uses and words derived from latin go from -us to -i! Simple! If only it were true, and if only words with latin derivations didn't then originally start from greek and vice versa.
Octopuses and Octopi
It's octopuses - because it's from the greek (you can tell it's greek because Octo means 8 in greek, aparently, and greek words go from "-us" to "-uses") wheras cactus become cacti, because it's latin - having said that, biologists (and purists) would probably say octopodes.
Here are some others:
abacus -> abacuses, crocus -> crocuses, locus -> loci, fungus -> fungi
So now you know!