4 . Running is often tiring and a lot of hard work, but nothing beats the feeling you get after finishing a long workout around the track.

But while it’s long been believed that endorphins (内啡肽) —chemicals in the body that cause happiness—are behind the so-called “runner’s high”, a study suggested that there may be more to this phenomenon than we previously knew.

According to a recent study published by a group of scientists from several German universities, a group of chemicals called endocannabinoids (内源性大麻素) may actually be responsible for this familiar great feeling.

To test this theory, the scientists turned to mice. Both mice and humans release high levels of endorphins and endocannabinoids after exercise. After exercising on running wheels, the mice seemed happy and relaxed and displayed no signs of anxiety. But after being given a drug to block their endorphins, the mice’s behavior didn’t seem to change. However, when their endocannabinoids were blocked with a different drug, their runners’ high symptoms seemed to fade.

“The long-held notion of endorphins being responsible for the runner’s high is false. Endorphins are effective pain relievers, but only when it comes to the pain in your body and muscles you feel after working out,” Patrick Lucas Austin wrote on science blog Lifchacker.

Similar studies are yet to be carried out on humans, but it’s already known that exercise is a highly effective way to get rid of stress or anxiety. The UK’s National Health Service even prescribes (开药 方) exercise to patients who are suffering from depression. “Being depressed can leave you feeling low in energy, which might put you off being more active. Regular exercise can improve your mood if you have depression, and its especially useful for people with mild to moderate (中等的) depression,” it wrote on its website.

It seems like nothing can beat that feeling we get after a good workout, even if we don’t fully understand where it comes from. At least if we’re feeling down, we know that all we have to do is to put on our running shoes.

5 . The United States is a sports-loving nation. Sports in America take various forms: organized competitive struggles, athletic games, hunting and fishing. Most sports are seasonal, so that what is happening in sports depends upon the time of year.

Baseball is the most popular sport in the US. It is played throughout the spring and summer, and professional baseball teams play well into the fall.

Football is the most popular sport in the fall. The game originated as a college sport more than 75 years ago. It’s not the same as European football or soccer. In American football there are 11 players as well on each team, and they are dressed in padded uniforms and helmets because the game is rough and injuries are likely to occur. It is still played by almost every college and university in the country.

Basketball is the winner sport in American schools and colleges. Like football, basketball originated in the US, many Americans prefer it to football because it is played indoors throughout the winter and because it is a faster game. And the famous NBA games are considered the best of this game.

Other sports attracting a great many people include wrestling, boxing and horse racing. Although horse-racing fans call themselves sportsmen, the exactness of the term is questionable, as only the jockeys who ride the horses in the races can be considered athletes. The so-called sportsmen are the spectators, who do not come primarily to see the horses race, but to bet (打赌) upon the outcome of cach race. Gambling is the attraction of horse racing.

6 . Parents whose children show a special interest in a sport feel very difficult to make a decision about their children’s careers. Should they allow their children to train to become top sports men or women? For many children it means starting schoolwork very young, and going out with friends and other interests have to take a second place. It’s very difficult to explain to a young child why he or she has to train five hours a day, even at the weekend, when most of his or her friends are playing.

Another problem is of course money. In many countries money for training is available from government for the best young sportsmen and women. If this help can not be given, it means that it is the parents who have to find the time and the money to support their child’s development and sports clothes, transport to competitions, special equipment, etc.

Many parents are worried that it is dangerous to start serious training in a sport at an early age. Some doctors agree that young muscles may be damaged by training before they are properly developed. Professional(专业的) trainers however, believe that it is only by training young that you can reach the top as a successful sports person. It is clear that very few people do reach the top, and both parents and children should be prepared for failure even after many years of training.