[PW] So, when IS ceramic begonias?

[John Franklin]

In Diana Wynne Jones' story "Little Dot," there are three questions  
asked with apparently nonsense answers. A quick Google search assures  
me that the first pair:

"Why is a mouse when it spins?"
"The higher, the fewer."

is not something which Ms. Jones made up herself. (One of the results  
from Google says that the question was originally a reference to a  
common part of a steam engine, which is a pleasing notion.) The third  
question also has a reference to a real-world phenomenon, although  
I'm not sure if the exact question and answer has ever been asked  
before. (I won't reproduce it here because the combination of  
question and answer gives away the plot.) That leaves the middle  
question, which brings up nothing on Google. Does anyone recognize  
it? The exchange is:

"When is ceramic begonias?"
"Chocolate herrings are impure."

(For those wishing to read "Little Dot", it was recently published as  
part of the collection "Unexpected Magic".)

(Yes, I could probably write to Ms. Jones and ask outright. But  
that's less fun that trying to track it down without her help.)

-John Franklin