Rebecca's #5 baby tooth is on its way out, hanging by half a thread. She won't pull it, but is content to wiggle it about with her tongue, letting it lie completely horizontal, sticking out of her mouth.
In real life it's an impressive sight, eliciting either cringing (a visiting college student) or admiration from her audience. Becca's having a ball. In this case youth is most definitely not wasted on the young. :-)