"Doctor, you're doing it again."

"Doing what, Miss Grant?"

"Singing," Jo said. She looked up at her tall, elegant Doctor. "You were singing again. You always sing when you're working. Why?"

The Doctor set his beaker of bubbling purple liquid down on the lab table and tuned to her. "My dear Jo," he said, "I'm not singing. I'm speaking. It's the language of the universe."

"Sounds like singing," Jo replied.

"And what is singing?"

"Ummm... pretty sounds that go well together?"

"Right, at least we should hope so." The Doctor selected a book from one of the crammed shelves and flipped through its worn pages. "And what is sound?"

"...Vibrations?"

"Correct," the Doctor said, turning the book over to her. "Now read this-" he pointed at the open pages.

"'If you want to find the secrets of the universe, think in terms of energy, frequency and vibration.' Nikola Tesla." Jo looked up, confused.

"My old friend Nikola is right. It truly is the music of the spheres. Now, watch this reaction, I must go talk to the Brigadier."

As the Doctor walked away, Jo thought over what the Doctor had said. She then poked her head out the door and shouted "That was Pythagoras!"