Empty Cells in Contingency Tables

Nothing is not, said he,
The absence of anything.
Why so?, said she.
Because, said he,
Recall Aristotle?
Well no, said she.
It's so subtle, said he.
There are two varieties
Of empty cells, you see,
Empty of necessity
And empty accidentally, said he.
Uh?, said she,
I don't get it.
It's Nothing, said he,
Forget it.