Here's why:

* Anaerobe: An organism that can live without oxygen.

* Facultative: Meaning it can do something optionally, in this case, using oxygen.

Examples of Facultative Anaerobes:

* Yeast: Used in baking and brewing, they can ferment sugars to produce alcohol in the absence of oxygen.

* Many bacteria: E. coli, for instance, can switch between aerobic respiration (using oxygen) and anaerobic fermentation.

* Some human cells: Muscle cells can switch to anaerobic respiration during intense exercise when oxygen supply is limited.

Important Note: While facultative anaerobes can survive without oxygen, they typically grow and reproduce more efficiently when oxygen is available.