How do we know Changes?

"Change" is convenient and useful word. But What's Change means ? How do we know what Change is? I think that we can know "Change" by keeping touch with something or someone. When we have been studied,read some book or newspaper,magazine long time , then we can recognize the Change.Also when we have been kept company with many friends for a long time,at least ten years or more,we can know peoples' change.