"Why is it called a sixty-nine?"
The Master sighed. "Must you read over my shoulder?"
"I was just curious, is all. Trying to work out the numerical significance - "
"Of my pornographic fanfiction? Really, Doctor, you're handling my imprisonment worse than I am. Now lie down."
"I'm not tired, to the contrary, I - "
"Lie down, and I'll show you."
The Doctor looked dubiously at him, but obeyed. The Master lay down opposite him. "My head is the loop of the six, yours is the loop of the nine."
"How depressingly obvious. But convenient," the Doctor added, and leaned his head forward.