As a stand-alone or as part of the intended unit, this is a valid investigation of what causes condensation to occur.  By limiting the amount of air around a cold cup of water and comparing it to one out in the open, they find that the moisture comes from the surroundings. Not only can this be used in a physical science class, it can be used when studying the water cycle in your earth science class. Make sure to consult the downloadable lab manual for scientific background and a unit overview.