Parents won't let me get a ferret,how do I convince them?

I had 2 hamsters, 2 pet mice, and have a bunny that me and my mom share but they won't let me get a Ferret.They had a ferret when I was 2 (but I don't remember him) and they said it was the best ferret you could get but they won't let me get one. I am 12, I took/take good care of all my pets. Please Help!!!