A test tube is a glass tube with one end open and the other end closed. The closed end is rounded. Test tubes are used to hold small samples. They are primarily used for qualitative assessment and comparison. A common place to see these is the biochemistry lab. When a large number of samples need to be tested and compared, test tubes are used to make this easier. They are also easily capped with a rubber or glass stopper.
They are generally held in a test tube rack specifically designed for the purpose. If the test tubes become unsafe to touch with bare hands (whether due to heat or another reason), test-tube tongs can be used to move them.
Never heat a capped test tube