Its versus It's

So I'll admit it: I sent an email yesterday and I used the wrong "its." I was thoroughly embarrassed when I realized my mistake, and so I decided to put up a post explaining the difference for all of us and to encourage me to proofread more carefully next time!

So what is the difference? Let's start with the basics:

it's: With an apostrophe, it is a contraction, either "it is" or "it has."
e.g.: It's too bad I didn't proofread that email!

its: Without an apostrophe, it is a possessive pronoun, that is, a word that replaces a noun and shows ownership.
e.g.: Too bad email can't proofread its own text.

Still having trouble? The easiest way to figure it out is to try substituting in "it is" each time you go to use "its." If "it is" works, then you want the apostrophe. If "it is" doesn't fit, then you don't want the apostrophe.

e.g. It's (it is) a good thing I will proofread this post; it would be bad if the post had its (it is? doesn't work) content wrong!