"I told him, Fluffy's a piece o' cake if yeh know how to calm him down, jus' play him a bit o' music an' he'll go straight off ter sleep --"
-- Hagrid (PS16)
Hagrid gave Harry a carved wooden flute for Christmas his first year at Hogwarts (PS12). It made a sound like an owl hooting.
Hagrid let it slip to Harry that the only way to get past the three-headed dog Fluffy in the forbidden third floor corridor was to play music and put him to sleep, a secret he had also shared with a stranger in Hogsmeade who gave him Norbert the baby dragon (PS14, PS16).
Since Harry thought Professor Snape was about to steal the Philosopher's Stone while Dumbledore was away, he decided to sneak out using his Invisibility Cloak, then use the flute to put Fluffy to sleep.
When he, Hermione and Ron entered Fluffy's lair on the third floor, the dog was awake but a harp was lying on the floor, left there by Professor Quirrell, not Snape. Ron figured that as soon as someone stopped playing music, the animal must wake up again, so Harry began to play his flute and "from the first note the beast's eyes began to droop" (PS16). He then gave the flute to Hermione, who continued playing while Harry and Ron went through the trapdoor where the Devil's Snare plant was the next obstacle to the Stone.
- In the Sorcerer's Stone movie, Hagrid is seen playing "Hedwig's Theme" on a large wooden flute, but it isn't included in Harry's Christmas presents (PS/f).
- When the children enter Fluffy's lair, Quirrell has left a large harp playing by magic and Harry never uses a flute. Fluffy wakes up and the kids barely escape his three heads.