I don't think I would make a good architecture teacher however I spoke to my father about the first exercise I would do.

Have everyone in our class draw a line and then ask one of the students the following: "Each of you drew a line. What's special about your line? How does it stand out? How is it different from all the other student's lines?"

Then I would show them a line that I drew before and say "This line is parallel to the page. It is drawn in the middle. It's ends are an even distance from the top and bottom of the page. I measured it. It took 8 minutes to draw."