The name of the blog comes from the post quoted below, the original post. As I'm never sure if it's apparent what my younger son was talking about, I'm going to mention at this point that he was confused and thought we were talking about anemones. Which are, indeed, sort of purple things.
Older son: What’s an enemy, Mummy?
Me: Hmm. Well, it’s sort of somebody you don’t like and who doesn’t like you.
Older son: Oh.
Younger Son (in an authoritative tone): NO, an enemy is a sort of purple thing.