He is credited with the discovered that the yellow toy whistle found in the breakfast cereal Cap'n Crunch emitted (with slight modification) a tone at precisely 2600 hertz -- the same frequency that was used by AT&T long lines to indicate that a trunk line was ready and available to route a new call. This would effectively disconnect one end of the trunk, allowing the still connected side to enter an operator mode.

Although the Cap'n Crunch whistle is the origin of his pseudonym, he did not discover the whistle on his own. In actuality, he learned of whistle from some blind phone phreakers.

"I don't do that. I don't do that anymore at all. And if I do it, I do it for one reason and one reason only. I'm learning about a system. The phone company is a System. A computer is a System, do you understand? If I do what I do, it is only to explore a system. Computers, systems, that's my bag. The phone company is nothing but a computer."

-- From Secrets of the Little Blue Box by Ron Rosenbaum, Esquire Magazine (October 1971)