It's a bird! It's a plane! It's is short for "it is"! But its simply owns something — it's soooo possessive. Its is the possessive form of "it."

It's (with an apostrophe) is always short for "it is" (it's so fun) or "it has" (it's been nice knowing you). But, you might ask, don't apostrophes show possession, as in "teacher's pet"? Well, yes they do, but not necessarily with pronouns. Often an apostrophe is used to show a contraction, like in it's or who's. Here are some examples of it's:

"I'm astounded by people who want to 'know' the universe when it's hard enough to find your way around Chinatown," (Woody Allen)

"It's Raining Men" (The Weather Girls)

Without the apostrophe, the possessive pronoun its is less popular. It's not often an "it" owns something; it's usually "his" or "hers." But if a table is lopsided, one of its legs might need to get fixed. Don't call a person an "it" unless you want to get into a fight, so save the possessive its for things and give "his" or "her" to people. An animal has a gender, too, but it doesn't seem to mind, as long as its food dish is full. Here are more possessive its:

"Chopper 2 was over the scene at 44th Street between 5th and 6th Avenues where a black vehicle was seen tipped on its side." (CBS News online)

"Constant company wears out its welcome." (Irish Proverb)

In a nutshell: It's is always a contraction, so if you can replace it's with "it is" or "it has," then keep that apostrophe on there. It's appropriate! If you try to replace its with "it is" or "it has" and it doesn't make sense, don't use it. Its meaning will shine without that pesky apostrophe.