The definition of a Wissenslücke

I read this amusing post over at The Yellow Duckpond illustrating some of the perils of being an English teacher.

It demonstrates perfectly that you can know what something is, describe it so that everyone else knows what it is and yet have no idea what it's called or, come to that, what it's used for.

Fortunately, the correct name appears in the comments (and I'd like to find anyone who can tell me they knew that before reading the piece).