Candy, I really don't understand how you could miss it - it's so obvious! wink

More seriously, this highlights a problem that so many of my students had - and all my students were mature ones - average age about 50!
As a guide I would tell them that, if you could substitute "his" for "its" without losing the meaning (whatever it did to the accuracy) of the statement, then it was possesive and did not need an apostrophe. If you could equally use "it is" for "its" then it was a contraction and therefore did need one.
I, quite sensibly, didn't go into metalinguistics to examine the possible use of "its" as a plural form!
(E.g, How many "its" appear in the following sentence? "It doesn't matter if it hits it or misses it.") [ans = 4]

I'm immortal until proven otherwise