How do you correctly spell its?

Answer 1

That depends on how you are using it. :)

It's = use apostrophe for the contraction. This means "It is."

Its = use no apostrophe for the possessive. This indicates that "it" owns something.

Answer 2

It depends on what you're trying to say.

It's = "it is," as in "It's going to rain later."

Its = "belonging to it," as in "I have a dog and this is its dish" "

The way to be sure is to replace the word with "it is" or "it has" - if the sentence makes no sense when you do that, then you spell the word as "its" - if the sentence makes sense, then "it's" is correct.