Possible Duplicate:
Why doesn't “its” have an apostrophe?
Generally, there is an apostrophe when someone possesses something:
That's Gerald's cat.
Gerald's cat is dead.
But when it comes to the possessive of it, its is correct, and not it's.
The common house cat is endeared for its ability to land on its feet.
Why is this? If Gerald's is correct, why is it's not correct?