Mosquito biting an iron bull

So Sahn

This matter of hwa-du practice can be linked to a mosquito bitig an iron bull. The mosquito does not try to figure out, "Do I do it this way, or that?" Its needlelike mouth pressed firmy on the impenetrable, it risks body and life, in one moment penetrating through with its whole being
The Mirror of Zen. The classic guide to Buddhist Practice by Zen Master So Sahn, case 17, p. 30.