Since everyone with a computer is a graphic designer by default, we might as well know the correct terminology.
X-height is the height of the lower case “x” compared to a capital “X” of the same typestyle.
Have your students look through magazines or at webpages, at the type in headlines. Compare how the X-height changes from font to font (typeface to typeface). Then discuss the subjective differences between the various X-heights; is one more pleasing than another?