If someone is saying something you disagree with, speak up. Silence, or polite smiling and nodding, is equal to agreeing with them in their minds. Imagine you are going on about how awesome Nickleback is and everyone around you is either silent or nodding absently. You are going to assume your opinion is perfectly normal, and even agreed with by most people around you.
This is how abhorrent ideas and harmful beliefs have spread around us. Because we don’t contradict them. We are too worried about shaking the boat, or don’t want to make someone feel bad, or we want to be polite, or think we don’t know enough about the subject–the same old fourteen reasons why not to speak up.
Well, it’s your job as a human to speak up. So stop holding back. Disagree!