A test tube is a kind of https://simple.wikipedia.org/wiki/Laboratory_glassware">laboratory glassware, composed of a fingerlike length of https://simple.wikipedia.org/wiki/Glass">glass tubing, open at the top, usually with a rounded https://simple.wikipedia.org/wiki/Lip">lip at the top, and a rounded 'U' shaped bottom. They range in size from a couple https://simple.wikipedia.org/wiki/Inch">inches to several inches long, from a few https://simple.wikipedia.org/wiki/Millimeter">millimeters to a couple https://simple.wikipedia.org/wiki/Centimeter">centimeters in https://simple.wikipedia.org/wiki/Diameter">diameter. They are designed to allow easy heating of https://simple.wikipedia.org/wiki/Sample">samples, to be held in a https://simple.wikipedia.org/wiki/Flame">flame, and often are made of expansion-resistant glasses, such as https://simple.wikipedia.org/w/index.php?title=Borosilicate_glass&action=edit&redlink=1">borosilicate glass (known by brand-names such as Pyrex and Kimax).