Most people have never thought about the question.
Ask an acquaintance if they believe the earth to be round or flat. Typically, they'll inform you that it's a fact, not a matter of belief.
Then, you say, "Oh right. I didn't know! When did they prove that then...?"
Endure some vague comments about space adventures, sailing around the world, people not trusting what they are told... etc.
Realise how little most people have thought about anything they/everyone know.
It's fun. I do it frequently.
Edit: People like this are the majority by a far margin.