"All" statements are very strict and correspondingly hard to prove true and easy to prove false. Since "Some" statements are very loose and flexible the opposite holds. "Some" statements are easy to prove true and hard to prove false. "Some Americans own guns" can be proven true with a single example. Find one American gunowner and you have proved the statement. However, consider how hard it would be to disprove the statement "Some Americans can recite the entire Constitution of the United States from memory." Even finding millions of Americans who could not perform this remarkable feat I would still have fallen short of proving that the statement was false. The reason is that if there is just one American who can do it, it is true.
So "Some" statements are easy to prove true. One example of something that is a member of both the subject and the predicate categories proves the statement true. However, so long as I have not examined every single member of the subject category, no matter how many negative instances I find, I will not have proved the statement false. Another mistake to watch out for is this. Sometimes people will a negative example will actually prove the particular statement true. It does not. For example, does finding one unfriendly person prove that some people are friendly? No, you need a friendly person to prove that some people are friendly.