The word "the heart of a child in a fist" comes from a famous saying of Mencius: "A man's adult must not lose the heart of a child." Here, "fist" refers to sincerity and profundity, "pool" refers to newborn babies, and later refers to people who love and are loyal to the motherland. So "pure heart" refers to loyalty and love for the motherland, as well as firm belief and courage.