People constantly get confused with it's and its. I see the wrong useage in professional memos and signs all the time, and once on CNN (smack that editor!).

So here it's:

it's = contraction of "it is", e.g. "it's confusing, true, but still"
its = possessive of it, e.g. "CNN should be ashamed of its mistake"

So only use an apostrophe for the contraction, *never* for the possessive. This goes against the way all other possessives work, but it's the only way to avoid confusion with the contraction.