Mineral oil and the material the test tube is made out of have a very similar index of refraction. When the test tube is placed in the mineral oil, as the oil fills the test tube, the tube "disappears." The demonstration usually used with this set-up is as follows: A test tube is placed in the mineral oil before the lecture. During the lecture, the lecturer smashes another test tube in an envelope and pours it into the oil and stirs it, then reaching in with a pair of tongs is able to pull a whole test tube out of the oil, telling the students it was put together with a magic fluid. Upon completing this trick, it is then shown that the test tube, when placed back in the mineral oil, disappears. It should also be shown that another test tube, placed in water, does not disappear because the test tube does not have the same index of refraction as water.