HELP!!!!!!!! how do i convince my mum and dad to let me buy a hamster?????

i am 13 nearly 14 and i really want a hamster my mum dose not know how much i want one! i have done presentations wrote letters, posters and leaflets! and still they say NO! i had one dog and 3 cats when i was younger but i was too young to look after them and they where to old to play with PLEASE HELP !!!!!!!!