This is a book on mental growth written by Yin Jianling for teenagers, in the form of letters, sharing the secrets of adolescence, leading teenagers to better understand themselves and the world. Fifteen letters cover topics from self-awareness to interpersonal communication, from the warmth of family to the budding of love, from poetic literature to real choices, depicting the entanglement and hesitation, the greenness and beauty of youth. Big friend Yin Jianling guides the way for teenagers with her delicate and spirited writing, accompanying them step by step towards a splendid tomorrow.