What I Learned Today

"That" vs. "Which"

It all depends on whether the clause is essential or not. Essential clauses use that, nonessential clauses use which.

For example:

The applesauce that I made this weekend filled up my pantry shelves.
(the clause "that I made this weekend" is essential to the sentence--because it indicates that I'm not talking about the applesause I made last weekend)

The applesauce, which I made this weekend, filled up my pantry shelves.
("which filled up my pantry shelves" is not necessary to the understanding of the sentence)

Here's another clue: nonessential clauses, which should use which, are always surrounded by commas.

Grammar is sexy.