Test tubes are a common piece of laboratory equipment consisting of a finger-like length of glass or clear plastic tubing, open at the top and closed at the bottom. They are widely used by chemists to hold, mix, or heat small quantities of solid or liquid chemicals, especially for qualitative experiments and assays. Test tubes are usually placed in special-purpose racks, clamps, or tongs. They are also used as homes for microorganisms when people want to culture (grow) them, and as collection devices for things like blood samples. In general, test tubes are used to mix chemicals, hold liquids, heat liquids, grow bacteria, and collect or store bodily fluids, like blood and semen. Their round bottom and straight sides minimize mass loss when pouring, make them easier to clean, and allow convenient monitoring of the contents. The long, narrow neck of a test tube slows down the spreading of gases to the environment. Some test tubes are made to accept a ground glass stopper or a screw cap, and they are often provided with a small ground glass or white glaze area near the top for labeling with a pencil.