Singing is his hobby.
"Hobby" 来源于英文单词 "hobbyhorse"，最早记录于13世纪中期，原意指儿童玩具马，后来逐渐演变为指个人爱好或兴趣。
例句：
He spends his free time singing in a local choir. (他把空闲时间用来参加当地合唱团唱歌)
Singing is her hobby, she always sings in the shower. (唱歌是她的爱好，她总是在淋浴时唱歌)
He has been singing as a hobby since he was a child. (他从小就有唱歌作为爱好)