It's Wagging Its Tail (it's and its)

 
It has been said that "its" is the most frequently misspelled word in the English language. That is because it is often confused with "it's," the contraction for "it is" or "it has." These two words are similar to "your" and "you're," which were the subject of yesterday's post, and the same principles apply.

The first word is possessive, showing ownership of something. Since "it" is a personal pronoun, no apostrophe is used in the possessive form.
That old car has served ITS purpose.

The second word is the contraction for "it is" or "it has," and the apostrophe is taking the place of the missing "i" or "ha." Remember that the apostrophe is a sign that something is being left out.
IT'S time to get a new car. (IT IS time....)
IT'S been ten years since we've bought a new car. (IT HAS been ten years....)

Usually mistakes with these words involve overusing the contraction "it's"; in fact, some people seem unaware of the existence of "its." Remember that the one WITHOUT the apostrophe shows ownership, and the one WITH the apostrophe means "it is" or "it has."

        sentences using "its"                               sentences using "it's"
The city is proud of its schools.             It's been years since I've visited Boston.
My puppy licked its paws.                      It's been a wonderful experience.

To be sure, whenever you write "it's" in a sentence, read the sentence aloud substituting "it is" or "it has," and see if it makes sense.