Yunyan Sweeps the Ground (BS21)

KOAN:

Once, when Yunyan was sweeping, Daowu said to him, “Too busy.”
Yunyan said, “You should know that there is one that is not busy.”
Daowu said, “If so, then there’s a second moon.”
Yunyan held up the broom and said, “Which moon is this?”

—Book of Serenity, Case 21