DISCLAIMER: I don't take any responsibility of the results of your actions! This guide is provided solely as a tip for developers, as an incentive to develop fixes remove this little harmless 'vulnerability' from their sites. It exists on most forums. If you use it with bad intentions, prepare for the consequences! Most likely this will result in at least a warning.
OK, enough of that
male cow. What if you want to say that somebody kicks a

ss, but the forum administration believes such word can only be used as an insult? As only whole words are detected ('a

ss' is censored, but 'asXs' is not), you can put anything anywhere in the middle of the word. Why not put then in something
invisible?
That's right! All hail BBCode! You can put in a img link to a
nonexistent image, e.g. [img]http://110mb.com/blank.png[/img]. (Sometimes the tags without any link are enough, but don't rely on that.) Now look at the source:
a[img]http://110mb.com/blank.png[/img]ss
gives you
a
ss
,
while
a
ss
gives you
donkey
(or whatever is the replacement in that particular forum).
That's it! Have fun, and one last thing: don't try this at home kids!