AUNT JENNY AND THE APPLE PIE
After two years of trying to teach first-year college students how to distinguish between “that” and “which,” I found a method that works. When the discussion of restrictive and non-restrictive clauses and comma placement fails to bring complete enlightenment, I give them an example. Basically I tell them a story, one about Aunt Jenny and her apple pie.
“It’s Thanksgiving, and your whole family is over. You and your sister are in charge of dessert. While most of your relatives like ice cream after Thanksgiving dinner—something light after all that turkey and mashed potatoes—Aunt Jenny, who is your favorite aunt, is old-fashioned, and she still likes apple pie. So you baked her one.
“The main course is over, and you and your sister are in the kitchen getting out the dishes for dessert. You turn and tell your sister that the ice cream is in the freezer and the hot fudge is in the refrigerator. Then you add that ‘The apple pie, which is for Aunt Jenny, is in the oven.’”
I write that sentence on the board.
“Now, imagine a different Thanksgiving. Same guests, same deal, except that everyone wants apple pie. You have made four apple pies. However, Aunt Jenny is diabetic, so you made hers without any sugar. Since the pies all look alike and you don’t want them to get mixed up, you put hers into the oven first. As your sister is getting out the dishes, you tell her that the three pies on the counter are for everyone else, but ‘The apple pie that is for Aunt Jenny is in the oven.’”
And I write that sentence down.
Then we look at the two sentences, the second one below the first, and I ask the class, “Do you see the difference?” And they do!
At this point, you can explain the grammatical distinction in detail and they will understand it. I tell the students that in the first sentence, the speaker simply says “The apple pie” because there is no question which apple pie—there is only one. The bit about it being “for Aunt Jenny” is simply additional information. In the second sentence, the subject is “The apple pie that is for Aunt Jenny”; that is what is in the oven. You can’t eliminate the fact that it’s for Aunt Jenny without scrambling your meaning, which requires you to distinguish Aunt Jenny’s pie from the others.
I always give them one more example (involving, say, “The car that has a flat tire is in the driveway”), but it simply serves to confirm what the students have now gotten. For some reason, the story of Aunt Jenny’s pie nails it. The students stop making that/which errors, which previous classes have hitherto always continued to stumble over.
I don't know why it works, but it does.