Things go into her mouth.
River is a year old when she discovers this, and spends a day putting Things into her mouth - marbles, light pens, apple cores, dishwasher detergent. She remembers that particular day because it's the only time in her life she remembers seeing her mother cry.
Things still go into her mouth.
"Not that," and the Captain takes an automatic drill out of her hand. "You don't wanna chew on that, darlin'."
"Chew, to masticate. To grind food into digestible particles." She shakes her head. The drill would have been cold against her teeth, it would have tasted metal, like blood. "I wasn't going to chew it."
"Well good then." He puts it out of her reach. "You behave while we buy this jzhan-mah and I'll buy you an ice planet. Deal?"
River's face lights up. She remembers the treat; it was a riddle, a puzzle, something she could not figure out. She couldn't figure out how to put it in her mouth easily. It delighted her. "Please," she tells the Captain.
"All right."
She leans over his shoulder, where he's looking at a used lazer saw. "that one's faulty," she tells him. "Buy that instead."
It is a rusty Thing. The Captain is dubious. "You sure?" but River says,
"the handle wants replacing, but it will cut true. I promise not to chew it."
He's sold.