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 . 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 .