You do not really understand something unless you can explain it to your grandmother.

Albert Einstein (1879-1955) German-American physicist

Alternate versions:
  • "If you can't explain something to a six-year-old, you really don't understand it yourself."
  • "If you can't explain it simply, you don't understand it well enough."
No source found. The quote is frequently also attributed to Richard Feynman.  It is likely based on a similar quote by Ernest Rutherford.

The closest reference to it can be found in Ronald W. Clark, Einstein: The Life and Times (1972):
To de Broglie, Einstein revealed an instinctive reason for his inability to accept the purely statistical interpretation of wave mechanics. It was a reason which linked him with Rutherford, who used to state that "it should be possible to explain the laws of physics to a barmaid." Einstein, having a final discussion with de Broglie on the platform of the Gare du Nord in Paris, whence they had traveled from Brussels to attend the Fresnel centenary celebrations, said "that all physical theories, their mathematical expressions apart ought to lend themselves to so simple a description 'that even a child could understand them.'"

More discussion of this quotation here.