D., professor of pediatric urology at
Wake Forest University Baptist Medical Center, in Winston-Salem, North Carolina. "The bladder is a muscle just like any other," he says. "If she's been doing the potty dance all day, every day, she's constantly contracting her bladder." This makes the muscle thicker and the bladder's capacity smaller. Pretty soon, it'll start contracting with less warning and more force--and your kid ends up with wet pants.