Einstein, it is said, was once asked by a layperson to explain how radio works.

"Well," he said, "first I need to explain the telegraph. The telegraph is like a giant cat. The cat's head might be in New York, and the cat's tail in London. You pull on the tail in London, and the cat meows in New York. That's the telegraph.

"The radio is just like that. Only there's no cat."