The word "mellow" also describes people and moods. In that sense, to be mellow is to be gentle, relaxed, and happy. And if a person or a mood mellows, it becomes gentle and relaxed.
A mellow voice is smooth and rich. A mellow smile is gentle and relaxed. And when you're in a mellow mood, you're happy and restful: maybe even sleepy.
Mellow parties are laidback and chill. You might hear mellow piano music there: soft, quiet, slow, relaxed music. And you might enjoy mellow conversation there: quiet, happy, pleasant conversation.
Remember: mellow things, like fruit and music, are sweet and soft; mellow people and other things are very chilled-out and relaxed.
Idea 1: "(Someone) used to be wild and crazy, always (doing something loud or silly), but (he or she) has mellowed with time."
Idea 2: "Our voices are mellow and our laughter is soft as we remember (some wonderful event or time from long ago), and how (something particularly happy took place)."
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