All the people know that exercise is important. We all need to exercise. Doctors say it is good for us. It makes your heart and body strong. Children who often exercise are more alert . It is healthy for the mind and the body. This means they do better in tests and schoolwork than those who don't exercise. There are many ways to do exercise. You can walk, run, swim, skate, or play ball games. Make sure you exercise in the following ways: 1. You have to like what you're doing. 2. Exercise enough, but not too much. It's best to exercise twice each week. Thirty minutes each time is enough. 3. Try all kinds of things until you find one, two or even three sports _ for you. Lots of people choose to exercise at fitness centers  . Why? Because there are a lot of sports equipment there. The equipment will help exercise your arms, legs and other parts of your body to make you healthy. Some people buy sports equipment for their homes. But it is very expensive. Exercising can be fun. Friends can exercise together at a fitness center, or they can play sports together outside. How do you exercise? It's best to exercise  _  a week and  _  each time.
once; thirty minutes two times; an hour twice; half an hour once; an hour