I don't know where this came from, and it is so short that I almost didn't post it - but then I decided this can be a collection of drabbles. MINOR SPOILERS for Face the Raven.
You may now continue.
A girl with too many heartbeats looked at a girl with none.
"I wish I could give you some of my heartbeats so you could feel alive again." said the girl with too many heartbeats to the girl with none.
"I feel alive traveling with you around the universe." said the girl with no heartbeats. "But I cannot have any more. That would unravel the fabric of the universe, tear apart time. The Doctor would not like that. I promised him not to destroy the universe. Until we return to the Time Lords, I must have no more heartbeats."