IF you are the type of person who likes to run and play, how do you know that you are fit? Well, there is really no one way to measure physical fitness. Usually, it is based on a combination of these things: How strong the body is; how long the body can keep going during exercise; how well the body can bend and move, and how much of the body is made up of fat. This measurement can be taken at a doctor's office or a health club. All of these things must be considered. For example, a person who can run eight kilometres, but who cannot lift 10 kilograms is not considered fit. A person who can lift 90 kg but who cannot run around the block is not fit, either. For young people, staying active is the best way to ensure fitness. If you like to run and play, you are probably doing well. Youngsters who come home every day and watch television, or who never play outside or participate in sports, probably are not fit.