Here's why:

* Mitochondria are the powerhouses of the cell. They are responsible for cellular respiration, the process that breaks down glucose (sugar) and converts it into usable energy in the form of ATP (adenosine triphosphate).

* When a cell needs more energy, it will increase the number of mitochondria. This allows it to produce more ATP to meet its energy demands.

Think of it like this: if you need to do a lot of work, you'll need to eat more food to get the energy you need. In the same way, a cell that needs to do more work needs to "eat" more glucose and produce more ATP to fuel its activities.