High school students typically learn how to use the average atomic masses of the periodic table to find molecular weights. Normal curricula does not scratch the surface of the amazing story behind what goes into making and calculating the average atomic masses. Here, we investigate the origin of the average atomic mass, and explore new dimensions of chemistry, such as isotopic abundance, the use of carbon as a standard, and the mass defect (missing mass) which makes atoms lighter than they “should” be. All these factors go into making some of the otherwise confusing average atomic masses, which appear on the periodic table. For example, why does a mole of oxygen weigh 15.999 grams, and not just 16? We answer this question in this lesson!