The word 自动 is formed by combining two meaningful characters: 自 (zì), which means "self", and 动 (dòng), which means "to move" or "to act". When placed together, they create a conceptual compound where the meaning of "self" directly modifies the action of "moving", resulting in the core idea of "self-moving" or "acting by itself". This fundamental sense then naturally extends to the modern meanings of "automatic" or "voluntarily", as both concepts originate from the principle of an action initiating from within the object or person, without external force.