I adopted a rat from Petco a week and a half ago, and adopted a companion for her a week ago. The first rat isn't tame, is scared, and will not come out of the cage when I roll it into the bathroom for playtime with both rats. She is friends with the second rat who is tame, loves attention, and loves coming out of the cage. I feel I'm making no progress with the first rat, who sometimes will allow me to pet her if I'm petting the second rat, but continues to bite my hand hard when it's near. How can I get her to stop the biting?