"Vernon, look at the address -- how could they possibly know where he sleeps? You don't think they're watching the house?"
-- Aunt Petunia (PS3)
When a letter arrives addressed to Harry in the cupboard under the stairs, Uncle Vernon and Aunt Petunia realize that Dumbledore knows how they’ve been treating the boy. In response, they move Harry into Dudley’s second bedroom, much to Dudley’s chagrin. The next letter, which arrives to following day, is addressed to Harry in “the smallest bedroom.”