The Sisterhood of the Traveling Pants by Ann Brashares
Carmen got the jeans at a thrift shop. They didn't look all that great; they were worn, dirty, and speckled with bleach. On the night before she and her friends part for the summer, Carmen decides to toss them. But Tibby says they're great. She'd love to have them. Lena and Bridget also think they're fabulous. Lena decides they should all try them on. Whoever they fit best will get them. Nobody knows why, but the pants fit everyone perfectly. Even Carmen (who never thinks she looks good in anything), thinks she looks good in the pants. Over a few bags of cheese puffs they decide to form a sisterhood, and take the vow of The Sisterhood of the Traveling Pants... the next morning, they say good-bye. And now the journey of the pants - and the most memorable summer of their lives - begins.