Take an aluminum pot and fill with water. Put this on top of any heat source and let the water heat up. Place the test tubes in the hot water to heat. If heating the liquids in the test tubes to a specific temperature, make sure students put the thermometer in the test tube, not the water. For smaller scale work, use the bottom half of an aluminum can. Many times, the water bath will be much larger than the test tubes and they might fall over, into the water. Devise methods to prevent this. You might clamp the tubes to the side with clothespins, attach parallel wires to the container to rest the tubes in between, or punch holes in a flat piece of plastic to put over the top of the water.