A petri dish is a small dish shaped like a cylinder. Scientists use it to grow cells from animals, fungus, and diseases so they can study them. They are usually made of glass or plastic. The glass ones can be used again if they are heated at 160°F. Sometimes people fill them with agar, which helps cells grow. These are called 'agar plates'. Many people use them in science classes at school. They have lids so germs in the air can not get into them, or "contaminate" them, and ruin the experiment.