Mitochondria are the organelles of the cell involved in breaking down sugars in order to release energy. The energy is then used for work. "Work," in science, is defined as moving something from one place to another. You have no doubt been studying cells and their organelles in class, and have been shown "typical" plant, bacteria and animal cells. Most cells, however, look nothing like the "typical" diagrams, and are very specialized for the type of job they perform. Some cells have far more mitochondria than others, for example, and this reflects how much work those cells do. Skin cells, for example, do not "do" much except form a layer that protects what is underneath. Muscle cells, though, need a lot of energy in order to do their job of contracting and moving parts of the body. They have many mitochondria. The cells can also produce more of a particular type of organelle when they have the need, and the thigh muscle of a long-distance runner needs to contract/work a lot. These cells have many mitochondria, reflecting this need.