How do I know what I know?

                                    That's a really good question to ponder on. Of course, my parents have helped me learn some things, most of the basic things. Then, everything from middle school till now has been learned through experience. But what about the things I couldn't learn? The things that I just know (or assume) to be true. Things that can't necessarily be explained through science or reasoning. For example, how do I know that doors are supposed to open? My parents never came up to me and said "Look Indigoe, it does this!"
I learned and know they open from seeing it open several times. Or for example, how do I know that people on TV shows are not behind the screen? I actually don't understand that whole technical process but I've known for forever that those people couldn't possibly be in my house. I guess you can say that's just common sense. The more I think about this question, the more confused I get about how exactly I know what I know. It's really hard for me to think about something that I wasn't taught or that I haven't experienced. That seems to be the only two ways in which I've learned what I know today. No one comes out the womb knowing information. We all learn and experience. Even for things that don't make sense or things that our parents go "Because!" to. I'm guessing since it's repeated to us over and over again and since the human race as whole pretty much follows these rules that we register it in our minds of common sense that this has to be right. Thus, we no longer question these things. I'm getting lost in my thoughts now.