I'm about to bake raps from scratch like Carl Sagan

from Nice Peter (Ft. Lloyd Ahlquist) – Einstein vs Stephen Hawking Lyrics on Rap Genius

Meaning

Carl Sagan was ‘ a famous theoretical physicist and scientist, famous for claiming there were only 'billions and billions of stars’ (although he later said he was misquoted).
Hawking is also saying he makes his raps up in the moment, ‘from scratch’.
What does that have to do with Carl Sagan?
http://www.youtube.com/watch?v=7s664NsLeFM&feature=related

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