A Petri dish is a shallow, flat, cylindrical container made of glass or plastic, commonly used in laboratories. It has a lid that fits over the dish, creating a controlled environment for growing cultures of microorganisms, such as bacteria and fungi. The dish is typically filled with a nutrient-rich gel called agar, which provides the necessary food for the organisms. Petri dishes are essential tools in microbiology and are used for various experiments, including testing the effectiveness of antibiotics and studying the growth patterns of different species. They allow scientists to observe and analyze the behavior of microorganisms in a contained setting.