She's watching him carefully, because if there's one thing the Doctor's good at teaching someone it's how to keep secrets. She wants to get home, and she's not convinced this Captain Jack isn't her way to do that.

Whatever he's thinking, though, she couldn't say. At any rate, it's gone with a puff of smoke the second he starts laughing.

"Impressive," she intones, looking chuffed. "Has he ever let you drift in space outside the TARDIS?"