The true sign of intelligence is not knowledge but imagination.

You can't blame gravity for falling in love.

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

If we knew what it was we were doing, it would not be called research, would it?

When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That's relativity.

Men marry women with the hope they will never change. Women marry men with the hope they will change. And they are both disappointed.

Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe.

It is a miracle that curiosity survives formal education.

When you do nothing, you feel overwhelmed and powerless. But when you get involved, you feel the sense of hope and accomplishment that comes from knowing you are working to make things better.

If at first, the idea is not absurd, then there is no hope for it.