In modern society, many people find that they do not get very much exercise. They travel by bus, train, or car, and they can move up or down in elevators. They have machines that do most of the heavy work that was once done by people. However, exercise is important for staying healthy. A person who exercises frequently will be physically fit. There are different aspects of physical fitness. One important element of being physically fit is called cardio-vascular fitness. The word cardio-vascular refers to the heart and lungs. If one's heart and lungs are in good condition, then one can exercise at a steady pace without soon getting tired. To develop cardio-vascular fitness, it is important to perform some exercise that makes one's heart beat quickly. For example, a person can improve cardio-vascular fitness by riding a bicycle, by running, by swimming, by rowing, or even by walking quickly. Some people go to special places, called health clubs or gyms where they can use different exercise machines to develop their cardio-vascular fitness. But many people improve their cardio-vascular fitness by playing recreational sports, such as tennis or soccer, or by dancing vigorously. By having good cardio-vascular fitness, one is less likely to suffer from heart disease. Of course, there is one way to improve cardio-vascular fitness that does not involve exercise-stop smoking! Another important aspect of physical fitness is called flexibility. Flexibility refers to one's ability to stretch comfortably. For example, a healthy person should be able to touch his or her toes without bending the legs. People can perform various stretching exercises to improve flexibility. By being flexible, a person can avoid injuries that might otherwise affect their body. Yet another important aspect of physical fitness is the strength and endurance of one's muscles. Many people suffer from injuries that result, in part, from weakness of the muscles. Muscular strength and endurance can be improved by a variety of exercises. Some of these exercises involve lifting a weight, but other exercises simply involve pushing or pulling against the weight of one's own body. One example is an exercise called the push-up. In this exercise, a person lies on the floor, with the face pointing down. The person bends his or her arms, so that the palms of the hands are on the floor, next to the shoulders. The person then pushes with his or her arms, lifting the upper part of the body above the ground until the arms are straight. Before starting to do any exercises, it is a good idea to have a medical check-up. Some people do not want to exercise because they think it will be too much work. However, many people find that they feel very good when they exercise. Exercising can be an enjoyable and fun way to maintain health and fitness.