Microorganisms are tiny. They are so small they can only be seen with a microscope. Yeast is a helpful microorganism which makes bread rise. Bacteria is a microorganism which breaks down plants ...
In hopes of seeing why a peppercorn tastes peppery, Antonie van Leeuwenhoek (1632-1723) soaked one in water and put it under a microscope ... the first bacteria ever observed.