Ask a number of students to collect samples of pond water or scrapings from rocks in streams, the inner walls of an aquarium, or the inside of a stock watering tank. Numerous one-celled organisms, such as protozoans and algae, can be observed from such specimens, as can more complex tiny crustaceans.
Students can make wet mount slides and view one-celled, living creatures from these samples. Alternatively, set up a classroom demonstration viewing these samples using a teaching microscope attached to a video monitor.
If your budget permits, purchase live one-celled organisms from biological supply houses. Protozoans, whether ciliated, flagellated, or sessile, make excellent specimens for student observation. Examples include Amoeba, Paramecium, Euglena, and Hydra, among many others. Carolina Biological Supply also offers pre-stained living organisms that are excellent for teaching and demonstration purposes. Often a quieting solution, such as Carolina's Protoslo, is recommended.
Prepared microscope slides may also be used to view one-celled organisms, but student interest is keener with live materials.
Go through a mathematical exercise concerning the various sizes of spherical objects and their surface area-to-volume relationships. Using a golf ball, a tennis ball, and a basketball, have a student volunteer measure the radius of each object. Then determine the surface area and volume for each ball (surface area is 4(pi)r2, and volume is 4/3(pi)r3). Review with students why the golf ball would make a more likely candidate for a cell than would a basketball.