A stated fact can be proven as either true or false, an opinion can not.

"The sky is red."
This is something you incorrectly believe to be a fact, not your opinion. It's night. There is not a single speck of red in the sky. If you seriously believe it's red there is something wrong with you.

"I think/In my opinion the sky is red."
This is a true fact. The person who said this does indeed believe the sky is red. (Though if you want to be a smart ass you can argue that only the person who the thought belongs to can prove this). If it were sunset, there may be a little room for debate.

"Advent Children is a really good movie."
Well that just depends on your own definition of a good movie. To some people, a good movie just has to do it's job: keep your mind occupied. Others need additional requirements met before they can call it a good movie. So this is just an opinion that can't be right or wrong. Even if you do not understand the definition of the word "opinion" or "fact."