Look for a very old window, and note the lens effect in the bottom part of the glass. Glass has both solid and liquid properties; in fact, it is often thought of as a very viscous liquid. Over many years, its downward flow due to gravity is evidenced by the increased thickness near the bottom of the pane. Tie this observation of “plastic” behavior into our discussion of plate tectonics. Which parts of Earth behave plastically? Which parts are rigid? What do these ideas have to do with plate tectonics?