The word 开心 is formed through a vivid conceptual metaphor where the character 开 (kāi), meaning "to open", is combined with 心 (xīn), meaning "heart" or "mind". It visually and notionally pictures the heart/mind opening up, like a flower blooming or a door swinging wide, to directly express the resulting emotional state of release and unobstructed flow, hence its meaning of "happy" or "joyful". The formation is not literal but an artistic, figurative fusion of an action and its emotional consequence.