http://en.wikipedia.org/wiki/Chinese_water_torture
Chinese water torture is the popular name for a method of water torture in which water is slowly dripped onto a person's forehead, driving the victim insane. This form of torture was first described under a different name by Hippolytus de Marsiliis in Italy in the 16th century. Supposedly the torture in dripping water is the slow rate at which the water flows. The victim can almost predict when the next drop will fall and a sense of tension builds up. When the drop finally does fall, a sense of shock and relief follows, only to be replaced with more tension about the next drop. The release of tension (no matter how small it is) prevents the victim from withdrawing inside himself. As this does not require interaction on the part of the torturer it can be done continuously.
The American television program MythBusters had mixed success with the Chinese Water Torture myth. They proved that when the victim is restrained, Chinese Water Torture is in fact torture, but this is caused by the restraints. The required torture equipment (and involuntary restrictions on movement) is highly efficient even without adding the discomfort of the water drip. The water drip itself, without the equipment, is almost negligible. The show performed two experiments, one in which a victim was restrained and had water dripping on her head and another where the victim was not restrained but still had water dripping on his head. The restrained victim reported elevated levels of stress and anxiety, while the unrestrained victim reported a mild level of annoyance. MythBusters cast member Kari Byron was unable to handle being restrained while subjected to the Chinese Water Torture for more than a couple of hours before breaking down.
http://en.wikipedia.org/wiki/Chinese_water_torture