Why does my dictionary refuse to tell me the right words?

I love those random bits of information that turn up in my textbooks.
I guess if I ever had to enter a room I suspect to contain carbon monoxide and thought myself safe after I saw frogs hopping around, it *would* be useful to know that frogs don't get CO-intoxications because they don't depend on their haemoglobin for their oxygen-needs.

ETA: I'm pretty sure it says something (though I have no idea what) that I would choose Valentine's Day to post about frogs.