One common analogy for the mitochondria (singular mitochondrion) is a powerhouse, as mitochondria create energy for the cell. Mitochondria are sometimes called the furnace of the cell as well. Like a powerhouse or furnace, mitochondria take in basic fuel stuff and generate energy from it: a furnace generates heat energy, and a powerhouse generates electricity, whereas mitochondria generate ATP.
ATP is a chemical that is found throughout all the domains of life, from dandelions to amoebas to tuna fish, mushrooms and people. Food is made (as in plants) or eaten (as in animals) and ultimately broken down into ATP to create chemical energy for being alive. The process of creating ATP from food is called the citric acid cycle.
Mitochondria take in nutrients floating in the cytoplasm and break them down within their inner membranes using enzymes specialized for the task. Among the chemicals involved is the familiar citric acid, found in fruits such as oranges and lemons, but here involved in the splitting of molecules for energy generation. Once the ATP is made, it is removed from the relatively impermeable inner membrane by a chemical called ATP translocase.
There are multiple mitochondria within a single cell, and all of them work to generate the vital ATP. Cells that are more active contain more ATP, carrying their powerhouses within them for swift and efficient production of the energy they need.