Birds are good pets, they can wake you up by singing in the morning.
鸟是很好的宠物,它们早上的歌声可以将你从梦中唤醒。
and we have only to turn to that extraordinary discovery of echo-location in bats to see a case in which the voice plays a strictly utilitarian role.
我们只要看一看蝙蝠回声定位这一极不寻常的发现,就可以探究一下声音在什么情况下有绝对的实用价值。