This is one mistake I see over and over. Writers use it’s when they should use its. The confusion is understandable. If you’re writing a paper for a grade, though, and your instructor is picky about grammar and punctuation, it’s can take points from your grade.
It’s is a contraction. A contraction is one word that is short for two words. One example is don’t, which is short for do not. Another is can’t, which is short for can not.
It’s is short for it is. That’s (that is) the only proper use of it’s. If you can’t (can not) replace it’s with it is, because it wouldn’t (would not) make sense, then you shouldn’t (should not – do you get this contraction thing now?) use it’s in that sentence.
Its, with no apostrophe, is the possessive for it. A possessive is used when you want to refer to ownership or belonging. For example, you might say, “That is Sherry’s hat.” (Or, “That hat belongs to Sherry.”) “My dog’s bowl is empty.” (In other words, “The bowl that belongs to my dog is empty.”)
There’s where the confusion comes in. All of those possessives have apostrophes. However, for every rule in English, there is at least one exception. For possessives, its is one of those exceptions.
For example, you might say, “That train is taking its time to get here.” Try to substitute it is in that sentence. “That train is taking it is time to get here.” Obviously, that doesn’t make sense. But if you use it’s in that situation, that is what you are saying.
Here are some other examples:
“That car is really showing its age.” (That car is really showing it is age? I don’t think so.)
“This can of Coke has lost its fizz.”
“My textbook is missing its last two pages.”
When you’re about to write it’s, stop and ask yourself if you can substitute it is. If you can’t, then its is the word you want.