Here are David Foster Wallace’s solutions to the “that that that that that” puzzle. If you want to play along, you might want to click the preceding link and read the puzzle before viewing the solutions below.

Now:

Five
“He said that that that that that writer used should really have been a which.”

Six (using what DFW calls the “medial-question-mark-in-sentence trick”)
“He said that? that that that that that writer used should have been a which?”

But I prefer Nacho’s solution:

Eight
“Do you know that that “that” that that “that” that that Nacho bolded precedes is italicized?”

If you’re having trouble parsing that, check out his explanation.