Whitewater kayaking is the sport of padling a kayak on a moving body of water, typically a whitewater river. Whitewater kayaking can range from simple, carefree gently moving water, to demanding, dangerous whitewater. River rapids are graded like ski runs according to the difficulty, danger or severity of the rapid. Whitewater grades (or classes) range from I or 1 (the easiest) to VI or 6 (the most difficult/dangerous). Grade/Class I can be described as slightly moving water with ripples. Grade/Class VI can be described as severe or almost unrunnable whitewater, such as Niagara Falls.