Fill an ordinary latex balloon with helium gas and you know what to expect. Over the next day or two the volume will decrease noticeably as helium escapes from the balloon. So what happens when a latex balloon is filled with carbon dioxide gas? Surprisingly, carbon dioxide balloons deflate at rates as much as an order of magnitude faster than helium balloons. An investigation into the details of this phenomenon provides students with an excellent opportunity to apply the kinetic theory of gases and the ideal gas law, and it can also be exploited for a dramatic in-class demonstration of diffusion and the second law of thermodynamics.