The bond between childhood sweethearts is often very deep.
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This idiom comes from a poem by Li Bai (李白): 郎骑竹马来,绕床弄青梅 — 'He rode a bamboo horse, circling the bed playing with green plums.' It describes an innocent childhood romance.
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Picture green plums (青梅) and a hobby-horse (竹马) — two children playing together carefree. The image captures innocent first love.