The Curious One

Then Christopher got curious about evaporation, so we did a sort of an experiment.  We had seven little glass bowls, so we put a quarter cup of each kind of liquid we could find in our kitchen, and set them in the middle of the kitchen island to see what would happen. 

I thought I would make one bowl empty to be a sort of control, so that if anything wierd happened to any of them (like a moth fell in the coffee one the first day, but to my dismay, my husband plucked it out before I could photograph it), we could compare to the one that had nothing, but my scientist husband tells me that's not a control.  Rather, the oil, that we know wouldn't evaporate, would be a control.  My question is, though, that if we assume we know nothing about evaporation (which is where I'm coming from with a three-year-old), then how would we know to use oil as a control?  And if the water evaporates and leaves ittle white things behind it, how will we know how much of it is comprised of minerals and how much is lint from our kitchen (of course, how would I know about minerals, since we are only 3???)   Obviously I need a little help in the science education department.  I also screwed up the experiment by then merrily filling our previous "Control" bowl with Diet Dr. Pepper, which was (un)fortunate enough to miss the papparazzi and will remain the unappreciated, unphotographed substance.