What Problems to Solve - By Richard Feynman
http://genius.cat-v.org/richard-feynman/writtings/letters/problems
_A former student, who was also once a student of Tomonaga’s, wrote to extend his congratulations. Feynman responded, asking Mr. Mano what he was now doing. The response: “studying the Coherence theory with some applications to the propagation of electromagnetic waves through turbulent atmosphere… a humble and down-to-earth type of problem.”_