I'm a high school chem and physics teacher, participating in a recreational online chem course hosted by MIT. One of the practice questions in this course asked how many atoms a certain mass would contain at 300K. This got me thinking about mass-energy equivalence, and whether the atomic mass units listed on the periodic table are defined according to a specific temperature, analogous to the way other values are defined at STP. This would mean that the number of atoms in a sample of mass X would vary inversely with the temperature of X. My research led me to these conclusions: (and I know this is an incomplete understanding)...temperature is kinetic energy. Kinetic energy can alter the apparent mass of a system because gravity couples to momentum and energy, not just mass. Thus the mass may appear to change, but this is simply the aggregate effect of the (unchanged) mass and the kinetic energy. I found several sources that support this view, or at the very least, the statement that non-relativistic changes in temperature/kinetic energy will alter mass, although the values may be too small for us to measure with existing tools. However, the response from the class's forum, including an MIT student administrating the forum, is that "mass is NOT temperature dependent" and will in no way affect it. I'm not sure if I'm completely misinterpreting mass-energy equivalence, but this doesn't make sense to me.