I wish I could draw, but I can’t. Sometimes I draw very simple stick figures and sometimes those leave my students even more confused than before.
A few days ago, my students were doing a communicative activity. The book provided two sets of drawings of six people with small difference between them, the activity being to describe each person accurately enough that their partner would understand that the drawing was different. One of the people was wearing sunglasses on her head. One student said, “She has a glass on her head”. I asked “What has she got on her head?”. She replied “A glass”. I quickly drew these stick figures:
I repeated my question and the student answered correctly. It turns out that the sunglasses were not one of the differences between the two sets of drawings. (The same often happens with “She has a long hair”.)
Glass can only mean “the substance”, and “a glass” can only mean “a drinking vessel”. But “glasses” can mean “two drinking vessels” or “spectacles”. But in this context, “She has glasses on her head” is going to be interpreted as “spectacles”. In the context of a circus performance, we’d have to say “She is balancing glasses on her head”.