The human mind works at low efficiency. Twenty percent is the figure usually given. When, momentarily, there is a flash of greater power, it is termed a hunch, or insight, or intuition.

All of a sudden, space isn't friendly. All of a sudden, it's a place where people can die. . . . Many more people are going to die. But we can't explore space if the requirement is that there be no casualties; we can't do anything if the requirement is that there be no casualties.

I made up my mind long ago to follow one cardinal rule in all my writing — to be clear.

Tell me why the stars do shine, Tell me why the ivy twines, Tell me what makes skies so blue, And I'll tell you why I love you. Nuclear fusion makes stars to shine, Tropisms make the ivy twine, Raleigh scattering make skies so blue, Testicular hormones are why I love you.

Old people think young people haven't learned about love. Young people think old people have forgotten about love.

While he lives, he must think; while he thinks, he must dream.

Having no unusual coincidence is far more unusual than any coincidence could possibly be.