This afternoon, I walked into a classroom, in which my assistants were having their sessions with my students. I overheard one boy talked to his friend. He said, ‘I believe that every man has a secret!’. His friend replied, ‘How do you know?’, then he said, ‘I just know’.
That minute strucked me, because I never expexted a 9-year-old boy could say such thing. This student of mine is indeed a smart boy. On his first class, I was shocked by his ability of telling stories in English (his primary language is Indonesian) without stopping for 2 hours! He took one encyclopedia from my book shelf and then he told stories on almost all the animals inside the book. His teacher (my assistant) looked at me every now and then, because she was amazed by his ability. I did too, I was amazed by him!
At his age now, he is in period of concrete operation. In this period, a child can solve problems which are concrete or actual. They will find it hard to solve any abstract problems. Following image will help you understand more on cognitive developmental stages by Jean Piaget:

Back to the story about my student. If you noticed the beginning of this article, you can see that his understanding of having secret is not an abstract concepts, because he said that ‘I just know’. He knows that everybody is keeping secret, but he could not understand yet on why everybody must keeps secret.
I believe that he will turn to be a smart adult, who will fully understand the meaning of having a secret.