The look she gave him, even in the dark, was clear enough: she didn't believe for a moment that two Erics could ever be bad. But she didn't contradict him, either -- just nudged his arm lightly, as if to say, That was a good one. Next time, try making it more believable.
She sobered a little when he said he coudln't eat, couldn't sleep, couldn't.... die... "That is a great gift," she said quietly. "You are able to protect many, even through that which others could not." She knew something of that, and admired anyone with a similar gift. "Protect life," she murmured, the words clearly sacred to her. "Until death."
But the moment passed, and she was back to smiling again. "A TARDIS," she answered. "Time and Relative Dimension in Space. It's a space/time ship that looks like a big blue wooden box, and it has been our home for almost a year. You do not have one? I believe it belonged to a man who went only by his title -- The Doctor."