Mice used the same basic system for determining motion, but had the problem that the ball was in contact with the desk or mousepad. In order to provide smooth motion the balls were often covered with an anti-slip surface treatment, which was, by design, sticky. Rolling the mouse tended to pick up any dirt and drag it into the system where it would clog the chopper wheels, demanding cleanup. In contrast the trackball is in contact only with the user's hand, which tends to be cleaner. In the late 1990s both mice and trackballs began using direct optical tracking which follows dots on the ball, avoiding the need for anti-slip surface treatment.