Mitochondria manipulate oxygen in a way that liberates energy from foodstuffs. Without this niftily facilitating trick, life on Earth today would be nothing more than a sludge of simple microbes. Mitochondria are very tiny—you could pack a billion into the space occupied by a grain of sand—but also very hungry. Almost every nutriment you absorb goes to feeding them.
We couldn't live for two minutes without them, yet even after a billion years mitochondria behave as if they think things might not work out between us. They maintain their own DNA. They reproduce at a different time from their host cell. They look like bacteria, divide like bacteria, and sometimes respond to antibiotics in the way bacteria do. In short, they keep their bags packed. They don't even speak the same genetic language as the cell in which they live. It is like having a stranger in your house, but one who has been there for a billion years.