It was interesting to find someone schooled in astrophysics. Most of the folks he had met might understand the theory on their best day. Practice was another matter. It was important to keep up with this one. After all, he had gotten away.
The Doctor fumbled in the pockets of his overcoat. He retrieved a scrap of paper and a pen. He jotted something down and handed it to Spock. "It might be useful to find me in the future. If we work together, we might well crack the City's dirty little secrets. Here's my number, I assume you will know how to find me with it."
The scrap of paper had a number. An atomic number of a substance that only occured in the arton energy eminating from the heart of a TARDIS.
He began walking towards the time ship and called back "Want a ride? You might find the trip a little eye opening."