A test tube, also known as a culture tube or sample tube, is a common piece of laboratory glassware consisting of a finger-like length of glass or clear plastic tubing, open at the top, usually with a rounded U-shaped bottom. A large test tube designed specifically for boiling liquids is called a boiling tube.
Test tubes are available in a multitude of lengths and widths, typically from 10 to 20 mm wide and 50 to 200 mm long. The top often features a flared lip to aid pouring out the contents; some sources consider that the presence of a lip is what distinguishes a test tube from a culture tube. Some test tubes have a flat bottom; some are made so as to accept a ground glass stopper or a screw cap. They are often provided with a small ground glass or white glaze area near the top for labeling with a pencil.