Start from the bottom to the top of a piano, or keyboard, just with your left hand. Your pinky first, ring finger second, middle finger third, index 4th, thumb 5th. Each finger has it’s own key, once those key are finish, move on to the next set, till you reach the top. At that point you can reverse the order of fingers. It get really fun, when you try to do this as fast as you can.
