A fluid is a continuous, amorphous substance whose molecules move freely, and that has the tendency to assume the shape of its container. Also, a fluid is readily reshaped, pliable. The human body excretes fluids, e.g. urine or sweat. Replenishment of fluids, in this case water, is essential to life and well-being. Fluids are exchanged in sexual intercourse.
Spanking is a physical act that produces sweating. A spanking session may cause the spankee to become dehydrated. The spanker must take a proactive role in providing fluids. The spanker may "order" the spankee to drink water, which may reinforce the submissive position of the spankee.