Mesa Verde National Park， in Colorado， America， has many“cliff dwellings2”， places that are built into cliffs where people once lived. They look almost impossible to get to. Who lived in them， and why did they live there？
　　People lived there about 800 years ago. Americans call them the Ancestral Puebloans. They grew food on the mesa3， the flat land on top of the cliffs. They kept animals over the mesa to trade with each other. Why didn’t they build homes on the mesa？
　　One reason was the weather. Winters were getting colder and colder. The mesa was flat and open. There was no protection from the cold winds. Most of the cliff dwellings face south or southwest—away from the cold north wind. Another reason people didn’t live on the mesa is that there were more and more people to feed. Homes on the mesa would use land that was needed to grow food. Some people say a third reason was for safety. They say different groups of Ancestral Puebloans were fighting each other. The cliff dwellings were hard to get into， so they kept the people who lived there safe. People who study Ancestral Puebloans argue about this reason. Some people don’t believe it. They say the groups traded too much to have been fighting.
　　Everyone agrees that these people were very skilled builders， who used every bit of space in the cliffs to make their homes. It’s a really admiring4 thing.
　　美国科罗拉多州的梅萨维德国家公园有很多悬崖屋，那里曾经有人居住过，如今改建成了悬崖。表面上看不可能进得去。那么谁曾住过那里呢？为什么他们会住在那里呢？
　　有一群人生活在那里大概有800年了。美国人称他们为古吉普韦布洛人。他们在位于悬崖顶处平坦的平顶山上种植食物。他们还在平顶山上饲养动物，互相交易。为什么他们不把房屋建在平顶山上呢？
　　其一是因为气候。冬天天气越来越冷了。平顶山既平坦又空旷。没有抵御凛冽寒风的保护措施。很多悬崖屋为了避开北面的寒流而面朝南方或西南方。人们不居住在那里的另外一个原因是有越来越多的人要养活。平顶山的房屋会占用那些本要用于种植食物的土地。有些人说第三个原因是安全问题。古吉普韦布洛人的各部落之间过去常常发生斗殴。悬崖屋很难进去，所以他们要确保住在那里的人的人身安全。研究古吉普韦布洛族的人们对于这点却意见相左。有些人不相信这一点，他们认为部落间交易频繁，是不会发生斗殴。
　　每个人都认为那里的人是技艺高超的工匠，他们能利用悬崖上的每一点空间修建房屋。这点的确让人叹服不已。