From Men in Black
Why the big secret? People are smart. They can handle it.
A person is smart. People are dumb, panicky dangerous animals and you know it. Fifteen hundred years ago everybody knew the Earth was the center of the universe. Five hundred years ago, everybody knew the Earth was flat, and fifteen minutes ago, you knew that humans were alone on this planet. Imagine what you'll know tomorrow.
What's the catch?
The catch? The catch is you will sever every human contact. Nobody will ever know you exist anywhere. Ever. I'll give you to sunrise to think it over.