Imagine if, for some reason - maybe because of something he ate on a diplomatic mission - Jim’s come starts to glow in the dark, and they only find out after Bones has let him fuck his mouth in the middle of the night, and it’s all half asleep and sloppy, so half of his semen ends up in strings across Bones’ mouth and chin and neck.
And Jim just runs his fingers through the lines and paints smudged fingerprints across Bones’ skin before licking up what’s left, making his lips glow in the same way that Bones’ is as he licks his lips to gather up the overspill.