Testing grass for microbes. Hand setting up an experiment to study the presence of bacteria and other microbes on grass. Samples of grass are placed in sealed Petri dishes filled with sterile agar, a common growth medium for bacterial cultures. Sample A is boiled grass, sample B is fresh grass, and sample C is a control dish containing agar only. The likely result is that a wide variety of microbes will grow in dish B. Only organisms that were able to resist the boiling will grow in dish A. Organisms growing in dish C would indicate contamination of the agar by the experimenter.