its and it's - the difference
 
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

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

Its is like his and her.

His is used for a masculine possessor (owner). (These are his pies.)
Her is used for feminine possessor. (These are her flowers.)
Its is used for neuter possessor. (These are its footprints.)

Examples:

It's been 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.)

The reef 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. (Winston Churchill)

There is nothing in the world like the devotion of a married woman. It's a thing no married man knows anything about. (Oscar Wilde)

Whenever cannibals are on the brink of starvation, Heaven, in its infinite mercy, sends them a fat
missionary. (Oscar Wilde)

Constant company wears out its welcome.

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 colours (colors ) of its environment.

Read more about it's and its here.
 
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NEVER EVER WRITE IT'S (THE EASIEST WAY TO GET IT RIGHT)

Never write it's.

Instead of writing it's, write the full version (i.e., it is or it has). If you cannot (because your sentence does not make sense), then use its.
 

 
THE CONFUSION

As covered here, apostrophes are used to show possession. For example, the possessive form of dog is dog's (as in the dog's teeth).

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.

Many, including some very prominent companies, still get this wrong. An it's error by the Co-operative Bank is discussed in Grammar Monster's grammar court.
 
 
POSSESSIVE ADJECTIVES

The words his, her, and its are known as possessive adjectives. There are no apostrophes in any possessive adjectives. This is another 100% rule.
 

See also:

Glossary of easily confused words
Glossary of common errors
Glossary of grammatical terms
What are nouns?
What are verbs?