Is beauty only skin deep? Is it in the eye of the beholder? Or is it the consequence of social circumstances? Shaw's more interested in dealing with the big questions – like that last one – than with old saws. In Pygmalion, anything from a pair of boots to a bath to an expensive dress can tell us important stuff about a character, like their place in the world or their state of mind. They can reveal what might normally be hidden from view, or hide that which might normally be obvious. So appearances can be deceiving, and the trick is learning how to judge what is true and what is false. The thing is, it's not an easy skill to pick up.
In Pygmalion, we learn that even the smallest changes in appearance can have a great effect on how a person or thing is perceived.
In constructing Pygmalion around the classic rags-to-riches plot, Shaw fools us in the same way Eliza fools the upper classes. We expect the plot to resolve itself in the conventional way until, suddenly, we realize that the play we are watching will not resolve itself at all.