Tīng wán lǎoshī de huà, tā ruòyǒusuǒsī de diǎn le diǎn tóu.
After hearing what the teacher said, he nodded thoughtfully.
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history
From Tang-dynasty author Chen Hong's (陈鸿) 《长恨歌传》, describing Yang Guifei's spirit: 玉妃茫然退立,若有所思 — 'The Jade Consort stood in quiet retreat, lost in thought.'
grammar
Pattern: usually adverbial with 地 — 若有所思地 + verb (点头, 看, 沉默). Also stands alone after a subject to describe an expression: 他若有所思.