这个音怎么发？
　　Place the tip of your tongue between your upper and lower teeth. Don’t put it between your lips. Make the sound by forcing air through the opening between your teeth and tongue.Don’t vibrate your vocal cords.
　　将舌头放在上下牙齿的中间而不是双唇中间，使空气从齿间通过，不要振动声带。