A lot of fallacies are taken as widely known. It's way too easy to drop numbers and claims if they cannot be backed up with sources of valid nature. If it's such widely known truth, it should be easily to fabricate such source if asked. Hmm.
It depends on development. If you tell a toddler something surprising, he will not say "I didn't know that." He will say "No it's not." That is because it takes time to build a universe and what you have said does not exist in his universe. When he reaches a certain level of development he will learn that there are many things in the universe that he has not learned about, and then he will consider some new information as previously unknown. This is a heady topic called "Bicameral Mind" and some students get very involved in studying the concept. Here is one that goes off the deep end with a lot of stuff that might have been "previously unknown" to you: http://www.viewzone.com/state-of-mind/01.html
