Fundamental Commercial Photography

That's the class I'm currently enrolled in. During this semester we've shot many different things using various lighting techniques but I've found throughout the semester that my assignments were always focused around a common theme... Can you figure out what it is from the images below? (I shot these for my final project on digital so that I can have a color copy of them. The actual assignment was shot on large format B&W film.)

If you guessed GLASS, then you are absolutely right...and very, very observant too! But I suppose I can see why some people would think otherwise...