资料：Most young people enjoy some form of physical activity. It may be walking, cycling or swimming, or in winter, skating or skiing. It may be a game of some forms, football, hockey, golf, or tennis. It may be mountaineering. Those who have a passion for climbing high and difficult mountains are often looked upon with astonishment. Why are men and women willing to suffer cold and hardship, and to take risks on high mountains? This astonishment is caused probably by the difference between mountaineering and other forms of activity to which men give their leisure. Mountaineering is a sport and not a game. There are no man-made rules, as there are for such games as golf and football. There are , of course, rules of a different thing that it would be dangerous to ignore, but it is this freedom from man-made rules that makes mountaineering attractive to many people. Those who climb mountains are free to use their own methods. If we compare mountaineering and other more familiar sports, we might think that one big difference is that mountaineering is not a "team game". We should be mistaken in this. There are ,it is true, no "matches" between "teams" of climbers, but when climbers are on a rock face linked by a rope on which their lives may depend, there is obviously teamwork. The mountain climber knows that he may have to fight forces that are stronger and more powerful than men. He has to fight the forces of nature. His sport requires high mental and physical qualities. A mountain climber continues to improve in skill year after year. A skier is probably past his best by the age of thirty, and most international tennis champions are in their early twenties.But it is not unusual for man of fifty or sixty to climb the highest mountains in the Alps. They may take more time than younger men, but they probably climb with more skill and less waste of effort, and their certainly experience equal enjoyment. Mountaineering involves ___.