-- It's the reaction that only comes when you've truly given someone the perfect gift.
For 9-year-old Tristen Perez, that gift was a yellow lab named Bailey.
His mother, Gretchen Perez, told ABC News the family lost their beloved dog Aspen in late August. Her boys, Tristen and Tanner, 5, were heartbroken.
"We started thinking about the timing of a puppy and thought, 'Well maybe around Christmas,'" Perez told ABC News. "So we wrapped up a box, put it on the doorstep and rang the bell. It was supposed to be a sweet little surprise after school."
The video of Tristen's reaction to that "sweet surprise" has been viewed nearly 5 million times on the Facebook page of the organization that gave the Perez family their dog, Mid South Puppies. He is overwhelmed by emotion when he sees the pup.
Genny House, who owns Mid South Puppies with her husband David House, called Bailey's meeting with the boys "the most rewarding part of my job."
Perez said Tristen's reaction is typical of her "sweet, caring child." The Memphis, Tennessee, mom said she thinks people are reacting so strongly to the video because of the beautiful bond it shows.
As for Bailey, she's "doing great" in her new home.