If you can't explain what you are doing to a nine-year-old, then either you still don't understand it very well, or it's not all that worthwile in the first place.
No fairer destiny could be allotted to any physical theory than that it should of itself point out the way to the introduction of a more comprehensive theory, in which it lives on as a limiting case.
Let me tell you what I look like: pale face, long hair, and a tiny start of a paunch. In addition, an awkward gait, and a cigar in the mouth and a pen in pocket or hand.
Concern for man himself and his fate must always form the chief interest of all technical endeavors, concern for the great unsolved problems of the organization of labor and the distribution of goods-in order that the creations of our mind shall be a blessing and not a curse to mankind. Never forget this in the midst of your diagrams and equations.
The real problem is in the hearts and minds of men. It is not a problem of physics but of ethics. It is easier to denature plutonium than to denature the evil spirit of man.