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A typical test tube is long and narrow, with a curved base and flared top.

A test tube is a kind of laboratory glassware, composed of a fingerlike length of glass tubing, open at the top, usually with a rounded lip at the top, and a rounded 'U' shaped bottom.

They range in size from a couple inches to several inches long, from a few millimeters to a couple centimeters in diameter. They are designed to allow easy heating of samples, to be held in a flame, and often are made of expansion-resistant glasses, such as borosilicate glass (known by brand-names such as Pyrex and Kimax).