I have heard a lot about those rishis. I know these are categories or may be level/rank of rishis. "Rishis" are bottom level and "Saptarishi" are top level. But what made them any of these? I mean, If one has become Rishi then how he can become Maharishi or Brahma rishi or any other. And what's the difference between them? Any scriptures describes this concept?