You truly understand something..

.. when you can explain its essence to someone who has no clue what it is.

EINSTEIN was asked by his hostess at a social gathering to explain his theory of relativity. Said the great mathematician,
"Madam, I was once walking in the country on a hot day with a blind friend, and said that I would like a drink of milk."
"Milk?" said my friend, "Drink I know; but what is milk?"
"A white liquid," I replied.
"Liquid I know; but what is white?"
"The colour of a swan's feathers."
"Feathers I know; what is a swan?"
"A bird with a crooked neck."
"Neck I know; but what is this crooked?"
"Thereupon I lost patience. I seized his arm and straightened it. "That's straight," I said; and then I bent it at the elbow. "That's crooked."
"Ah!" said the blind man, "Now I know what you mean by milk!"

:) Pure Genius.