Who puts the blanket around Scout's shoulders and how does Atticus reach this conclusion? What is Scout's reaction when she hears this information?...

One cold night, Miss Maudie's house catches on fire.  Jem and Scout go outside to watch with the rest of the neighborhood.  The next morning, Scout realizes that a brown blanket is around her shoulders.  Atticus points out that the blanket looks unfamiliar and does not belong to them.  Jem confesses about the knot-hole gifts and about his mended pants.  Atticus does not understand everything that Jem is talking about, but he does realize that his children have been communicating with Boo Radley in a way.  He concludes that Boo was the one who put the blanket over Scout in an act of friendship.

Atticus tells Scout that someday she can thank Boo for covering her with the blanket.  She is shocked and her "stomach [turns] to water and [she] nearly [throws] up when Jem [holds] out the blanket."  She cannot believe that Boo Radley had gone right up to her the night before.