Remember gym class?  If you're a child of the 80's or 90's, you were probably part of a group of kids who had to start writing essays instead of running laps.  At least in Massachusetts, where I grew up, the education reforms demanded that kids learned more about how the body works while participating in phys-ed.

So you know there are 5 main components of fitness:  cariovascular endurance, muscular endurance, strength, flexibility, and body composition.