What am I missing? According to the ideal gas law, as volume increases, temperature increases. However I’m not sure I understand why. I certainly understand the motion of the molecules equals the kinetic energy of the molecules. The faster the molecules bounce about, the more thermal energy the system contains. Further, the larger the number of bouncing molecules contained within the system, the more thermal energy the system contains. But if you reduce the volume without changing the number of moles why would that change the internal energy. In fact, why doesn’t the gas get colder as you increase volume? PV=nRT tells me if we increase the volume, the temperature increases – which means the internal energy increase. But what if we increased the volume of my system to the size of our universe – wouldn’t it be awfully cold in my system? Aren’t I describing the space of our universe – near absolutely zero in temperature with just a relative handful of molecules? Further, if I reduce the volume why would that necessarily alter the temperature? Sure, it would increase the number of bounces per unit time, but since each bounce transfers its energy to its dancing partner, why would more bounce mean either more energy or less energy? I know I’m missing something obvious but I just don’t see it. This is classical physics, not quantum, so the answer should be apparent to me. It’s not.