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Will putting people's hands in warm water really make them wet the bed?

Go With the Flow

If the soaking hand prank works, then how does it overcome the mechanisms we develop in childhood to prevent bed-wetting (see sidebar)?

Our bodies store urine in the bladder, a muscle bag that retains liquid by relaxing and expels it by contracting. Outbound urine streams through a tube called the urethra. To help the process of retention and release, muscles squeeze the urethra shut at two points: the bladder neck, aka the internal urinary sphincter, and the external urinary sphincter, located a bit further down.

Medically, the trick to overcoming postoperative retention lies in convincing these sphincters to relax. Studies suggest that sitting in a warm water bath or placing a hot water bottle on the pelvis can do just that, likely through a temperature-based sphincter reflex [sources: Elsamra and Ellsworth; Kulaçoğlu et al.; Shafik].

"Heat and warm water is a common relaxation modality," says Valre Welch, a pediatric nurse practitioner and past president of the Society of Urologic Nurses and Associates.

"It encourages blood flow, and it's very soothing after genital surgery."

But would the same reflex hold true when dunking, not the affected area, but a hand or a few fingers? The answer is a firm perhaps, if the paw plunge is combined with a very deep sleep and/or the power of suggestion.

Research bears out the power of watery images or sounds to instill a sense of urinary urgency [source: MUCMN]. Moreover, it makes sense: The inhibition and release of the micturition reflex -- the contraction of bladder walls and relaxation of urethral sphincters in response to bladder pressure -- is controlled by the brain and nervous system.

As pressure builds inside the bladder, the rate of nerve impulses flowing from it to the spinal cord surges. Neurons in the spinal cord then send signals to brain areas called the pons and cerebrum, which make us feel a conscious desire to pee. If for some reason urination isn't in the cards, the brain RSVPs with a polite "no" and signals via the spinal cord for the bladder to inhibit the reflex. Later, when it's go time, the brain sends signals that activate the micturition reflex, and it's a potty in the USA [source: McKinley and O'Loughlin].

During a typical night's sleep, a full bladder kicks off like an abdominal car alarm. It needles us until we wake up and take care of business. But get a little too drunk, stoned or exhausted, and you might just ignore that alarm -- especially if the power of suggestion relaxes you further via a pan of warm water.

"It really does occur," says Welch. "It is a phenomenon of relaxation."