How do i make my parents let me keep the dog i found.?

OK so i came home one day with my friends after hanging out with them and this dog was where we were and i just fell in love with that dog. me and my friend named it because it didnt have a collar and no owner that we saw for atleast 5 or 6 hours so we named it Sadie. And when me and my friend came home we didnt let my parents see so we brought it to the side of our house and i ran inside and got it some dog food. and some towels and one blanket and water. so we let it and drink its food and water. then it layed down. finally my mom came to where the dog was and we looked at her. right when she came over there my friends dad drove up. so she left and my mom told me to leave the dog alone or she will take it to the pound so i left it alone. then it followed some boy home and i havent seen it but i know ill see it again sometime. so what do i do to make my parents think i can have it. i already have 2 small dogs and 2 cats but this dog was big. so please help me please i love this dog!!!