PLANET EARTH Deserts
A third of the land on our planet is desert. These great scars on the face of the Earth appear to be lifeless, but surprisingly none are. In all of them life manages somehow to keep a precarious hold. Not all deserts are hot. Fifty-mile-an-hour winds blowing in from Siberia bring snow to the Gobi Desert（戈壁滩） in Mongolia（蒙古，注意这里是指蒙古国，不是内蒙古）.
From a summer high of 50 degrees centigrade the temperature in midwinter can drop to minus 40, making this one of the harshest deserts（最严酷的沙漠） of all. Few animals can survive these extreme changes. Wild Bactrian camels（野生双峰骆驼）, one of the rarest mammals（最近学了一个词，mammoth猛犸象） on the planet. And perhaps the hardiest. Their biggest problem is the lack of water, particularly now, in winter, when the little there is locked up as ice.
Surprisingly, snow here never melts. The air is just too cold and too dry for it to do so. The sun's rays turn it straight into vapor. It evaporates. But it is the only source of water, so Bactrian camels eat it. Elsewhere in the world a camel at a waterhole can drink as much as 200 liters during a single visit. Here the strategy is to take little and often. And with good reason, for filling the stomach（填饱肚子，固定搭配） with snow could be fatal. The camels must limit themselves to the equivalent of just 10 liters a day.
