Ball mills are used to prepare fine powders and to induce alloying and chemical reactions via mechanical deformation. Thermal processes are also at play, thus knowing the temperature inside the mill is important, but it had rarely been studied. Thus when Jonathan McHenry measured the temperature increase of the milling balls in two laboratory mills as a summer project some 15 years ago, unique and very valuable information could be obtained. He used a simple calorimeter from the introductory laboratory but made careful corrections for systematic errors. It turned out that if different mills were run at similar milling intensities, the balls warmed up more, when oblique collisions and friction were involved, like in planetary mills, where temperatures over 200 °C were typical, while close-to-normal impacts, dominant in shaker mills, resulted in much less temperature increase.