We all know that exercise is good for us. But are there any types of physical activity that are especially beneficial for our health? Being physically fit means you have the energy and strength to cope with everyday tasks - from listening attentively in class and taking an active part in physical education lessons, to doing your homework well. To be physically fit, there are three areas which we need to work on: cardiovascular fitness, muscle strength and flexibility. Cardiovascular exercises Cardiovascular exercises help us do physical work for a long time without getting tired. It includes any activity that gets your heart pumping and the muscles using oxygen. Health experts recommend that we exercise for half an hour every day. The American Heart Association suggests that teenagers give their hearts a good workout for 20 minutes without stopping, at least three times a week. By doing this regularly, your heart will become stronger and better at supplying oxygen to every part of your body. Cardiovascular or aerobic (means 'with air') exercises not only improve heart function but also lower blood pressure, increase 'good' cholesterol, reduce 'bad' cholesterol and helps to cut down body fat. Examples of aerobic exercise include running, skating, swimming, brisk walking, tennis, basketball, dancing, kick-boxing, hockey, football, rowing, handball, skiing and skipping. Strength training The heart and other muscles in your body benefit from exercise, too. Strength training is all about building muscles; the more muscles you have, the more calories you burn. Having muscle strength means that you can lift and move heavy things. Different types of strength training build different muscle groups. For example, push-ups, pull-ups or rowing are useful for strengthening the muscles in your arms. Running, cycling, skiing and walking are good for building up leg muscles while crunches and sit-ups are great for toning and tightening stomach muscles. It's best to do these sorts of activities in repetitive sets. For example, start with one set of eight sit-ups and gradually move on to three sets of 12 sit-ups. This helps improve muscle strength and endurance. Flexibility exercises Can you easily touch your toes, or do you feel tightness and pain? Being flexible means that you can move your limbs freely without any pain. Great stretching or flexibility exercises include yoga, pilates, ballet, tai chi, front or side stretches, dancing, skating and gymnastics. Stretching regularly is a sure way to improve your flexibility. Calories shown on the table (left) are for a person weighing 54 kilograms. A person who weighs more burns more calories, so the same amount of physical activity can burn the same number of calories but more quickly. Put your trainers on Walking, running, and jogging are free; all you need is a pair of trainers. Exercising is a great way of socialising with your friends. Instead of meeting up in a fast-food place, catch up at the park or in the gym. Even though you'll be tired just after exercising, you'll feel more energetic later. Exercising can put you in a better mood. When you exercise, your brain releases a chemical called endorphin, which helps to make you feel happier. Exercise increases your metabolic rate - this is the speed at which we burn energy or calories. So, not only do we use up calories during exercise but the increased metabolic rate that occurs while we exercise will carry on for several hours afterwards.