根据短文内容回答下列各题。
The Horse Riding
Bob is a middle school student. His favorite thing is to play games. He eats a lot of junk food. As he stays up late at night, he often sleeps in class. When he is having a P.E. class, he feels very sad. Most of his classmates say to him, "Bob, you are too slow. You'd better play all by yourself." What's worse, when he is running in a relay race, his team will complain about his action.
One day he went to a doctor. "Doctor, I'm sad when I play sports with my classmates.
How can I become thinner and healthy sooner?"
The doctor told him to keep on exercising every day. Bob thought it was a good idea.
A month later, he came to the doctor again and complained about exercising. Pulling a long face, he came to the doctor's office.
The doctor asked him, "How do you exercise, Bob? Do you run or play basketball?"
Bob answered, "I don't run or play basketball. I go for horse riding every day. The result is that I gain weight while the horse loses weight."
The doctor laughed, "You shouldn't ride the horse. You should run with your horse every day."