Mars once was wet and fertile. It's now bone dry. Something bad happened on Mars. I want to know what happened on Mars so that we may prevent it from happening here on Earth.

When I grew up I assembled my role models à la cart. I wanted to be an astrophysicist. If I tried to find a role model who grew up in the Bronx with my skin color who was an astrophysicist, I would never have become an astrophysicist.

Artificial selection turned the wolf into the shepherd, and the wild grasses into wheat and corn. In fact, almost every plant and animal that we eat today was bred from a wild, less edible ancestor. If artificial selection can work such profound changes in only ten or fifteen thousand years, what can natural selection do operating over billions of years? The answer is all the beauty and diversity of life.

Do you realize that if you fall into a black hole, you will see the entire future of the Universe unfold in front of you in a matter of moments and you will emerge into another space-time created by the singularity of the black hole you just fell into?

The chances that your tombstone will read 'Killed by Asteroid' are about the same as they'd be for 'Killed in Airplane Crash.'

I don't see why there is no intelligent alien life in the universe.

Once upon a time, people identified the god Neptune as the source of storms at sea. Today we call these storms hurricanes.... The only people who still call hurricanes acts of God are the people who write insurance forms.