What is a characteristic feature of a test tube?

A test tube is specifically designed for laboratory use and is characterized by being narrow and closed at one end. This design allows for the mixing and heating of substances while minimizing the risk of spills. The closed end is essential as it retains liquids or gases, making it suitable for reactions or experiments where containment is crucial.

In contrast, other options do not accurately describe a test tube. The conical shape is typical of flasks rather than test tubes. While some test tubes can be used with lids or stoppers, it is not a defining feature of all test tubes as many do not have them. Lastly, measuring mass is not a function of a test tube; a balance is typically used for that purpose. Thus, the defining characteristic of a test tube relies on its structure of being narrow and closed at one end, which supports its use in various laboratory applications.