it's a small world idiom saying
这世界真小;真有这等巧事;无巧不成书
said to show your surprise that people or events in different places are connected
You know my old science teacher! Well, it's a small world, isn't it? 原来你认识我以前的科学课老师!嘿,这真是太巧了,不是吗?
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