After Saturday errand-running was finished, I dug out my old VHS tape of "Charlie X", dusted it off, and took a look. The flirtation in the rec was a put-on! There were other people in the room, and Uhura performed this parody of seductive singing, aimed at Spock who everybody knew was the least likely male on the Enterprise to respond. She even got a couple of laughs out of her audience. And all the time Spock had this wry little smile on his face. He knew he was the butt of the joke.