A common mistake, and one that drives teachers of all levels crazy, is the mix-up between the words "its" and "it's". While the difference may not seem significant to the average writer, using the words correctly can help you appear more intelligent and educated. Many people form first impressions simply by reading someone's writing, so you want your writing to be as accurate as possible.
"Its" is a possessive pronoun, meaning "belonging to it." The confusion arises because if you don't substitute the pronoun "it" for the noun, an apostrophe is used. For example, the bone belonging to the dog is "the dog's bone." The eraser on the pencil is "the pencil's eraser." Both examples use an apostrophe plus an "s" in order to attribute ownership. But "it" has a different case