From Meng Haoran's Tang-dynasty poem
《春晓》 (Spring Dawn). Full quatrain:
春眠不觉晓,
处处闻啼鸟.
夜来风雨声,
花落知多少 - the sleeper hears birdsong, remembers last night's storm, and wonders how many petals the wind took down. The gentle, half-rhetorical question is the poem's famous punchline.