A Zen student called Hsiang-yen went to dokusan with Kuei-shan Ling-yu (771-853) , the T'ang dynasty master, and Kuei-shan gave him a koan, of which over and over he was unable to see into it's mysteries.
Hsiang-yen decided that it was all too much for him and he would surrender. He went away and found a sacred site, the grave of the Sixth Patriarch of Chinese Zen, Hui-neng, and maintained it as a shrine. Day in and day out he had no thought about the world except his sweeping. Then one day, sweeping away, he swept a pebble into a bamboo grove beside the shrine. The pebble hit a piece of hollow bamboo and went "ping!" and he jumped up and down.
The "ping!" shook him to pieces and he said, "One ping! and I have forgotten all I knew!" and he composed a poem in his excitement: "Last year's poverty was not true poverty, this year even the wind can get through". Hsiang-yen was Enlightened