The human being represents a diverse habitat: We have microorganisms on the skin, on the head, on body hair, in the mouth, in the intestines - basically everywhere. They feed mainly on dead cell material, which we produce in large quantities every day. These microorganisms are mostly harmless or even beneficial for us, because as they live so extensively on us, harmful microorganisms can no longer find a place and have to find another habitat. This is called niche occupation. Some microorganisms that live in us, such as in the mouth or intestines, help us to decompose food and digest it.
Explanation for children: We have microorganisms on the skin, on the head, on the body hair, in the mouth, in the intestines - so actually everywhere. But they don't harm us, because they mainly eat dead cells, which we don't need any more. Microorganisms are good for us because they protect us from evil microorganisms. Some microorganisms that live in us, such as in the mouth or intestines, even help us with digestion.