“If you can't explain it to a six year old, you don't understand it yourself.”

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

“Many of the things you can count, don't count. Many of the things you can't count, really count.”

“A photograph never grows old. You and I change, people change all through the months and years but a photograph always remains the same. How nice to look at a photograph of mother or father taken many years ago. You see them as you remember them. But as people live on, they change completely. That is why I think a photograph can be kind.”

“Look deep into nature, and then you will understand everything better.”

“No, this trick won't work... How on earth are you ever going to explain in terms of chemistry and physics so important a biological phenomenon as first love? ”

“I don't pretend to understand the universe — it's much bigger than I am.”