The Sluice Gate

A mill received water through a sluice gate the miller controlled.

In heavy rains, the river ran fast. The miller could open the gate wide and grind quickly.

He didn't. He opened it only as wide as his millstones could handle.

"You're wasting water," a merchant said.

"I'm saving my mill. The stones can only spin so fast. Faster water doesn't grind faster. It just breaks things."