Unicellular organism
Almost all unicellular organisms are microorganisms but you can observe eukaryotes and many prokaryotes by using a compound microscope. Bacteria just appear as dark dots. To gather unicellular organisms for observation, one can place a cover slip on the surface of pond water, and leave it overnight. By the next morning, numerous unicellular organisms will have grown entire colonies on the bottom of the slip. They multiply fast: colonies can double their size in between 30 minutes and a few hours.