What is it with horror movies and small children singing creepy, slowed-down lullabies? Us's take on the trope comes in the form of young Adelaide whistling "The Itsy-Bitsy Spider" as she nervously makes her way through the hall of mirrors attraction at the boardwalk. At first it seems like her voice is echoing off the glass that surrounds her, but becomes clear that, no — someone else is singing an identical version of the tune somewhere in the room with her.
Later on in the film, we discover that Red and Adelaide actually originated as their opposites — the little girl living a normal life with her parents was knocked out and dragged down to the world of the Tethered underneath the hall of mirrors the night of her birthday by little Red, who switched places with her in the world above, with her parents none the wiser. After a bloody showdown between adult Adelaide and Red, the latter is stabbed and uses her last breaths to whistle the song one last time.
Cue the chills.