We were sitting around the living room the other night when my husband said he was a "pushover" for something. None of the kids had heard that metaphor before, so he explained what it meant, and where it came from. Then he had to explain that, while it originally had connotations of giving in easily to certain things, he was just using to show that he liked whatever it was he was talking about.
That led to a discussion of how metaphors change over time. My oldest son remarked, "They should call those metamorphors."