If clogging occurs, it is usually the result of an attempt to flush unsuitable items, or too much toilet paper. Clogging can occur spontaneously due to limescale fouling of the drain pipe, or by overloading the stool capacity of the toilet. Stool capacity varies among toilet designs and is based on the size of the drainage pipe, the capacity of the water tank, the velocity of a flush, and the method by which the water attempts to vacate the bowl of its contents. The size and consistency of the stool is a hard-to-predict contributory factor.