and if we allow slightly longer chunks:

Punctuate the following:
(four sentences)

"That that is is that that is not is not that that is is not that that is not that that is not is not that that is is not that so?"

(spoil your fun if you want to:)
"That that is, is. That that is not, is not. That that is, is not that that is not. That that is not, is not that that is. Is not that so?"

PS. That is from the same book of shut-in puzzles and other activities ("The Big Good Time Book" was its title) in which I came across (in 1947!) "Tom, where Tim had had "had," had had "had had;" "had had" had had a better effect on the teacher." See PaulB's post eleven (flat-view) entries above, and also http://wordsmith.org/board/showthreaded.pl?Cat=&Board=wordplay&Number=57545 from 2/22/02. But Paulb's was fist.