This question is a rather difficult one to answer, mainly because there is no real right or wrong answer to it. It could take a lifetime of thinking to properly answer it, but I don't have that long so ill do the best I can!
The first answer that immediately popped into my head when I first read the question was that "we cannot be sure we know something, we can't be sure we know anything, we can't be sure we know everything". I say this because if I was sure that I knew everything then I wouldn't have to question what I know, like I'm doing now. If people knew everything then there would be no need for them to have to think, they would not have to wonder, they would never be curious because they would already know the answer to the question, which they are asking themselves.
This, in my opinion, would create a world full of boring people who would know everything there is to know about everything.
There may be many highly intelligent people in this world but it is unthinkable for one human being to know the answer to every question asked to him/her. I believe the most intelligent people in this world do not know everything, but the way I see it, they know something about everything and everything about something.
Once I actually stopped and thought about this somewhat challenging question being asked to me, I realized that I can be sure I know some things. For example, I'm sure that my name is Jake. How do I know this? Well because everyone has been calling me by that name since I was born. It is written on my passport (a legal document where the truth must be stated), and also on...