This is one of the most common slip-ups, and happens occasionally to even the most seasoned of writers. People tend to sneak the apostrophe in for either form, perhaps because it’s occurs more frequently in writing, so it looks weird without one. Or it could be that they learned that every possessive form has an apostrophe.
- Look, Mom! Its a beautiful sunset tonight for spying on the neighbors.
- That horse just flattened it’s ears; stop calling it a donkey.
- The popcorn popper just blew it’s lid.
Its is possessive, now and forever. It cannot be anything other than a possessive construction. It’s always means it is or it has. It cannot be anything other than it + is or it + has. “It is a beautiful sunset, and the popper just blew its lid.” Grammarians will blow their lids if you keep messing this up.
If you aren’t sure whether you’ve used the right word, substitute it is or it has, and see if either of those work. If so, use the apostrophe. Otherwise, no apostrophe!