Imagine this：you are twenty-one years old and a promising graduate student at one of the top universities in the world．One day, your doctor tells you that you have an incurable disease and may not have more than twelve months to live．How would you feel? What would you do? Most of us would probably feel very sad and give up our dreams and hopes for the future．Here is what Stephen Hawking thought：
　　(There did not seem) much point in working on my PhD—I did not expect to survive that long．Yet two years had gone by and I was not that much worse．In fact, things were going rather well for me and I had got engaged to a very nice girl, Jane Wilde．But in order to get married, I needed a job，and in order to get a job，I needed a PhD．
　　Instead of giving up，Hawking went on with his research，got his PhD and married Jane．Nor did he let the disease stop him from living the kind of life he had always dreamt of．He continued his exploration of the universe and traveled around the world to give lectures．In 2002，Hawking visited China and spoke to university students in Hangzhou and Beijing．As his disease has disabled him，Hawking has to sit in his now-famous wheelchair and speak through a computer．He told the students about his theories and thoughts on some of the greatest questions：What is time，how did the universe begin，and what exactly are black holes?
　　Hawking became famous in the early 1970s，when he and American Roger Penrose made new discoveries about the Big Bang and black holes．Since then，Hawking has continued to seek answers to questions about the nature of the universe．In 1988，he wrote A Brief History of Time，which quickly became a best-seller．Readers were pleased and surprised to find that a scientist could write about his work in a way that ordinary people could understand．
　　In the book．Hawking explains both what it means to be a scientist and how science works．He tells readers how discoveries are made and how they change the world．Science，according to Hawking，is often misunderstood：people often think that science is about “true” facts that never change．Scientists，on the other hand，Hawking writes，know that their job is never finished and that even the best theory can turn out to be wrong．
　　A scientific theory is the result of the scientific method．Scientists look at the world and try to describe and explain what they see．First，they carefully observe what they are interested in．To explain what they have seen，they build a theory about the way in which things happen and the causes and effects．Finally，the scientists test the theory to see if it matches what they have seen and if it can predict future events．If what they are observing Can be tested in a practical way，scientists will use