A test tube is a small, cylindrical glass or plastic container used in laboratories for holding, mixing, or heating substances. It typically has a closed bottom and an open top, allowing for easy access to its contents. Test tubes are commonly used in scientific experiments, especially in chemistry and biology, to conduct reactions or store samples. Test tubes come in various sizes and can be used with different types of equipment, such as test tube racks for organization and pipettes for transferring liquids. They are essential tools for researchers and students, facilitating hands-on learning and experimentation in the sciences.