That vs. which
|Towards more picturesque speech™
“I fed the cat that sat on the mat.” [implied: ... and I let the other one, which sat in the armchair, go hungry].
“The cat, which sat on the mat, had halitosis.” [Its halitosis was not contingent on its location mat-wise; indeed there could be an entirely distinct minty-breathed moggy sitting on the same mat, for all we know.]
Lesson: Get a dog.