How to convince my parents to get two guinea pigs?

I have asked my mom a while ago and she said that i could after fair because i show animals at the fair. But i just asked her about it and she said no. I am responsible i have a lot of animals outside like horse and sheep and goats and cats and a dog. the guinea pigs would be inside or out on the patio. I have researched a lot and i know a lot of stuff like they need unlimited hay and they are herd animals so you have to get two or the one will get lonely. Do you know any other way to convince her? PLEASE HELP ME!!!!!