In case you hadn't realised, it's boob's we're on about. These little marbles under the skin just appear one day. They might grow to be very big or stay very small. They may droop, point north, south, east, west, be pear shaped, melon shaped or look as flat as a pancake!
Yours may even grow faster or slower than your friends.

They are there for a reason - if one day a girl decides to have a baby, they will produce and store milk for the baby. They look pretty good too! If somebody is having a go about them, ignore it or tell them to get a grip - they are usually just jealous girlies who wish theirs would hurry up and grow or boys who want to know what they feel like!