Students and the general public are often told that the chronology of ancient events is known with high confidence, but the methods used to determine how long ago an event occurred are usually not described or even mentioned. This gives the impression that the methods are either not important or that only scientists can understand them. Fortunately, many of the techniques are understandable if properly presented. Here one key method of dating ancient materials, argon/ potassium radioisotope dating, is described in detail. In addition, a dramatic example of its calibration is described using the geology of the Hawaiian Islands. Sample lessons used in a high school physics class are described and discussed.