There seems to be no reasonable way to measure the thickness of a narrow-mouth glass bottle. One can measure the outer and inner diameters of the mouth with a ruler or a pair of calipers and then calculate the thickness. However, this measurement might be interfered with by the threads at the mouth. Furthermore, it is uncertain whether the thickness at the mouth is the same as that of the body. An optical method has been proposed. It requires the alignment of an incident laser beam tangent to the bottle and the measurement of the angle of laser light refracted out of the bottle. Here, another method for measuring of thickness is presented. It is first tested on a transparent ring, because the thickness of the ring can be easily measured. The method is then applied to measure the thickness of narrow-mouth glass bottles.