Humans had many different kinds of relationships, Susan knew. Grandfather had insisted that they do their research when they’d decided to stay on Earth for a while, and she’d read the chapter "Sol 3" from Case Studies in Xenoanthropology aloud to him while he tried to rebuild the TARDIS’s temporal combobulator. Based on that basic data, it was easy for her to deduce the common assumptions about love and relationships in this specific society. Parents and siblings were a somewhat alien concept, but people seemed to accept it when she told them she lived with her grandfather. Friendship was a harder concept to define, and the word seemed to translate several different Gallifreyan words, but the basic principle made sense.
For romance she had no lack of data. It was everywhere—in books, in films, on the radio—and she felt she had made quite the study of it. She understood popular love songs, although she still cared more about the rhythm. She could smile sympathetically when her classmates complained about their parents fighting or admitted to a crush.
That didn’t, she eventually learned, mean she was any good at spotting it.
They had turned up on a space station, today, and Grandfather and Ian were talking to a scientist. It wasn’t that Susan disliked talking to scientists, as such, but she didn’t think she had much to learn from a twenty-eighth century Ganymedean biochemist. So she was happy to go with Barbara to try to find somewhere on the space station where they could all have a meal.
It was extraordinary, thought Susan, to see Barbara navigating the space station, getting directions from the computer terminals in the walls, nodding politely to hovering robotic staff as she passed them. Members of non-time-active civilizations (she’d quit saying "lesser species" a while ago) weren’t supposed to be able to acclimate like this—it was one of the many, many reasons one wasn’t supposed to interfere. But Grandfather’s theory seemed to be holding up to experience: the humans coped better than anyone at home would have expected. Indeed, even if Grandfather didn’t like to admit it, Susan wasn’t sure sometimes what they would’ve done without Barbara and Ian.
Of course, from a xenoanthropological point of view, just as interesting as the relationship between the Gallifreyan and Earthling travelers was the relationship between the two humans themselves. Susan knew that the two of them had not been enemies at school; they had indeed been cordial colleagues. At home this would have been a very high level of trust and respect, the sort of working relationship that took centuries to form. But Susan was less sure what it meant to a human, and she was curious.
"Barbara," she asked unsuspectingly, "how do you feel about Ian?"
She hadn't been consciously expecting much of anything, really. But she certainly hadn't been expecting Barbara to turn noticeably pink, look everywhere but at her, and say, "I—I care about him a great deal, but, Susan, I don't want you to get the wrong idea."
Susan blinked and put on her blandest face.
Barbara went on. "Ian and I are very close, and—what made you ask, Susan?"
"I was just curious," said Susan. "I wanted to know what friendship is like for humans."
Barbara turned pink again. "Oh."
"Did I say something wrong?" said Susan innocently.
"Well, it's—oh, dear, I don't know how to say—well, I thought you were asking if we were a couple."
"Are you?" said Susan.
Barbara turned pink again. "I—if we were on Earth, after all this, perhaps..."
Susan smiled to herself and pointed out a restaurant. She might not have much field experience, but she thought Barbara perhaps didn't even realize just how informative her observation had been.
It was important to gather a sufficiently large dataset, of course. She would have to find an opportunity to ask Ian.