They are used to control the motion of the rock - the granite stone that is 'thrown' down the ice surface. A game of curling is not played on smooth ice, as used for skating, but a pebbled surface. The ice surface is first cleaned and smoothed, then a fine spray of water droplets is sprayed over the ice. The droplets freeze to the ice surface, producing a slightly rough or 'pebbled' surface. The stone is thrown with a slight rotation either clockwise or counter-clockwise depending on the path the skip decides it should take - the name 'curling' refers to the curl of the stone as it slides down the ice. Stones are usually made of granite and are highly polished. The underside of the stone is machined slightly concave, which produces a fairly sharp edge where it contacts the ice. As the games progresses and the pebbling is worn away, the ice becomes faster and the stones curl less. Brooms are used to sweep the ice in front of the stone to affect its motion. Hard sweeping makes the stone carry further and reduces the amount it curls. Light sweeping does the same, but to a lesser extent. The sweeping action causes the ice to melt slightly, producing a thin film of water on the surface of the ice. This reduces the friction of the stone, as it does not cut into the ice as much. Sweeping with the old corn brooms hard enough to make a loud slapping sound took a a fair effort on the part of the sweeper. Some brooms would have a strip of leather inside to make them more effective. Older curlers would use brushes, because they required less effort to sweep. However, brushes have replaced the brooms almost completely over the past couple of decades.