A rubber stopper, also known as a bung, is a laboratory tool used to seal containers and prevent the escape of gases or liquids. It is typically made of rubber or another flexible material and comes in a variety of sizes and shapes to fit different types of containers.

Rubber stoppers often have a tapered shape, allowing them to be easily inserted into the opening of a container, and some may have a hole in the center to allow for the insertion of a thermometer or other instrument.

Rubber stoppers are commonly used in chemistry, biology, and other scientific fields for tasks such as sealing test tubes, flasks, and other containers during experiments or storage. They can also be used in industrial settings for sealing pipes and other equipment.