One day at group meeting, the question of "which vs. that" came up. Oddly, though, instead of talking about grammar, syntax, or meaning, people talked about Microsoft.
You see, Microsoft Word will tell you that all these sentences are wrong (the squiggly underlining in this screenshot indicates that Word thinks something is wrong):
Well, the sentence: "John likes cheeseburgers, which are fattening" is correct. Because "which" is the proper way to indicate that all cheeseburgers are fattening, and "which" takes that comma.
And the sentence: "John likes sushi that is fresh" is correct. Because "that" is the proper way to indicate that not all sushi is fresh, and that John only likes that subset of sushi that is fresh. And "that" does not take a comma.
[The mavens say that "that" introduces "restrictive" clauses, while "which" introduces "nonrestrictive" clauses.]
But Word? Word does not know the difference between sushi and cheeseburgers! Word just knows that "which" takes a comma, and that "that" doesn't. So if you don't know the rules, but just modify your text to make Word happy, you could easily go wrong. And that would be a pity.
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