A test tube is a small glass cylinder, closed at one end and open at the other, used by scientists to hold liquids and chemicals during experiments. Test tubes are usually about as long as your hand and as wide as your thumb, made of special glass that can withstand heat and won't break easily when chemicals react inside them.

Scientists use test tubes constantly in laboratories. A chemist might pour two different liquids into a test tube to see if they react or change color. A biologist might grow bacteria in a test tube filled with nutrients. Medical researchers use test tubes to test blood samples or study how diseases work. The small size makes them perfect for experiments that only need tiny amounts of material.

Test tubes sit in wooden or metal racks that hold them upright, since the rounded bottom won't stand on its own. When scientists need to heat something in a test tube, they hold it with special tongs or clamps and move it carefully over a flame, never pointing the open end toward anyone in case the liquid suddenly boils.