Proof of Truth?

How do we prove that something is true? This question has two sides to its' answer, one is complicated and one is simple. The simple answer here is that we just know that something is true such as 1+1= 2 because that is just the way it is, that is the way that we learned when we were young so we just know that it is true and that is the way it is. The complicated answer is that we can not prove that something is true. The reason being that we can not define the world and what it is. People say that truth is knowledge. Everyone has a different level of knowledge depending on how old they are and how much education that have completed. Some may say that 1 + 1 = 2. Most people agree with this because they have the knowledge of our number system. The only people who do not have this knowledge are really young children, mentally slow people, and some bums who have blown away all their brain cells. This is one of the more simpler examples of something that we know to be true. There are plenty of more complicated things out there that we do not know if they are true because we do not all have the same level of knowledge. What are some of these things though?