My students know I am a fan of word and phrase etymologies, so they routinely ask me from where words and phrases stem. One of my favorites is “happy as a clam” because it’s one that mystifies the students.
From what I have been told and read, the original phrase was “happy as a clam at high tide” when the clams are safe from their predators. As people often do, they shortened the phrase and it lost the obviousness of its origin, how it came to mean content.
Fun assignment I use:
I hand everyone a slip of paper with a cliched saying on it, and then I have the students compose a paragraph explaining the origin of the phrase. The students come up with some incredible mini-stories, fables, and explanations. Sometimes we read them aloud or post them for the class to read. I’ve even used them as practices for speeches by having students read them aloud to the class.
Once we even made it a contest using categories: logical explanation, humor, and fable. We would vote and then give out prizes. Fun times.
In addition, this can be an entertaining way to teach grammar or some other writing convention. Maybe the kids have to use three present participles, two past participles, a gerund, and two examples of alliteration. Whatever you decide, it can be a more motivating method for teaching normally dry concepts or language uses.
Afterwards, I usually tell everyone how the phrase actually came into being.