Don't listen the first answer, that's lying. Just explain to her that you've never had rats before and you just want to try it out!(if that's your case at least) If she likes animals at all, she may grow to like them too if you're lucky! Just tell her you'll clean the cage twice every week (you need to for two rats) and take the very best care of them! I'm not trying to put words in your mouth, but you need to really want them to convince her.
Ok, tell her that its for a project or something. Say that you need an a... If you don't want to do that, tell her that fancy rats are much different then sewer rats (VERY different) and that you would like to study their behavioral patterns and social norms between each other. :)