It's is short for it is or it has.
Its is the possessive form of it.
There is often confusion between its and it's. If you delve deeper
into this issue, you will see that there is good reason for the confusion.
However, if you just want to know what is right, the matter is very simple:
It's is short for it is or it has. This is a 100% rule. It cannot be
used for anything else. If you cannot expand it's to it is or it
has, then it is wrong.
Its is like his and her.
His is used for masculine things. (These are his pies.)
Her is used for feminine things. (These are her flowers.)
Its is used for neuter things. (These are its footprints.)
raining for a week, and now it's starting to snow.
(It has been raining....it is starting to snow)
It's one of
the hardest courses in it's history.
(The first it's is correct. The second should be its.)
I think the
company wants to have its cake and eat it.
(its - possessive form. This is correct.)
shark chases it's prey through the coral.
(should be its, i.e., the possessive form. You cannot expand this to it
is or it has.)
I'm astounded by people who want to 'know' the universe when
it's so hard to find your way around
Chinatown. (Woody Allen)
A lie gets halfway around the world before the truth has a chance to get
its pants on.
There is nothing in the world like the devotion of a married woman.
It's a thing no married man knows anything about.
Whenever cannibals are on the brink of starvation, Heaven, in
its infinite mercy, sends them a fat
missionary. (Oscar Wilde)
Constant company wears out
A frog can't empty its stomach by
vomiting. To empty its stomach contents, a frog throws up its stomach first, so the stomach is dangling out of
its mouth. Then the frog uses its forearms to dig out all of the stomach's contents and then swallows the stomach back down again.
A completely blind chameleon will still take on the
) of its environment.
Therefore, somewhat understandably, many think that the
possessive form of it should be it's. It seems to fit the pattern. To
make matters worse, there is some evidence that the possessive form of it used
to be it's. The word it's is used erroneously (by today's conventions)
throughout the American Constitution.