Usually, the dish is partially filled with hot liquid agar along with a particular mix of nutrients, salts and amino acids that match the metabolic needs of the microbe being studied (technically referred to as a "selective medium"). After the agar solidifies, the dish is ready to receive a microbe-laden sample (although to grow some microbes it is often necessary to apply the sample with the hot agar). Modern Petri dishes often have rings on the lids and bases which allow them to be stacked so that they do not slide off of one another.
As well as making agar plates, empty Petri dishes may be used to observe plant germination, or small animal behaviour.