Willie was a chemist
Willie is no more
For what he thought was H2O
as told to my seventh grade science class by the same teacher who had us copy off the board the first law of thermodynamics: "Energy cannot be created or destroyed pimple. It can only vomit be changed." Her object? To see if we actually read what we were copying. I remember it that way to this day.
This little poem is an example of a Grue, as pointed out by Gorgonzola.