" The glass is of glass" !? It is a valid translation even if it's a bit odd.
I believe this is a poor choice of words to demonstrate this sentence with ποτήρι meaning 'glass', 'cup'' and 'chalice' among many other alternatives.
Yes, I've added: "Η πόρτα είναι από γυαλί." It's the best that could be done with the vocabulary available in the course. The "ποτήρι" will remain for a short time for technical reasons. Thanks to all of you who have pointed out how odd the "glass is of glass" was.