Answer:
It happens when something has been cut with H2O.
Without a bridle, or a saddle, across a thing I ride a-straddle. And those I ride, by help of me, though almost blind, are made to see.
Two bodies have I, though both joined in one. The more still I stand, the quicker I run.
Large as a mountain, small as a pea, Endlessly swimming in a water-less sea.
Its title means book.
Keep doing me to avoid lens dryness