as soon as i figure out whether there’s any practical difference between ‘that’ and ‘which’ in a sentence, you’re all finished
“That” is if the clause specifies which one, “which” is if you’re giving extra information.
“She took the bag that contains the loot” means that she took one of several bags, in particular, the one that happens to contain the loot.
“She took the bag, which contains the loot” means that there’s only one bag you might be referring to, and that the fact that she took it is important because it contains the loot.
Also, there’s a comma before “which” but not before “that.”
you’re all finished
Re: the comma, my 10th grade English teacher taught us this rhyme: “commas, which cut out the fat, go with ‘which’ and never with ‘that.’” That’s a different usage of which, but it does remind me about the comma, so I wanted to share.