摘要
词数:506阅读难度:建议阅读时间:6分钟You know the feeling. It’s impossible to resist. You just need to yawn. A yawn consists of an extended gaping of the mouth followed by closing rapidly. In mammals and birds, a long intake of breath and shorter outlet follows the gaping of the mouth, but in other species such as fish,amphibians(两栖动物)and snakesthere is no intake of breath.
