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Some of you speak with certainty only in situations where you actually know the subject deeply and have understood all sides of the argument. But then there are those who speak with certainty in all circumstances regardless of what the actual knowledge base is. But certainty doesn't dictate truth and nether does it make an opinion fact.
Don't let the capacity of others to make false opinions appear truth fool you. Reason with them and let them realize that at the end of their day, its just an opinion, and like anyone else's, can be wrong.