I told them to think of a simile as a mathematical formula where "like" is the same as an "=" sign. So "My love is like a red, red rose," is abstraction=concrete example. They couldn't write an abstraction=abstraction, like "love looks like happiness," which a surprisingly large number would write.
Because I like to skip steps, though, and hurry them along, I also told them they should avoid obvious constructions, like "patriotism looks like the American flag," or "love feels like a wedding ring."
The slogan of the day, which I put up on the board, was "Subvert the ordinary!"
One of the kids raised his hand then said, "Do you mean like death smells like burnt cookies?"
I loved it.