A Clattering Discovery

I scrambled out of the living room and trotted all the way down the front hallway! Down on the wooden floor beside a pair of sneakers, a heavy brass key ring was lying quietly. I gave it an eager nudge with my paws—jingle, jangle! That made such a cheerful ring! But wait, nestled right between two ridged keys was a flat, shiny metal tag shaped like a tiny padlock. I pressed against the cool brass with my cardboard sign, nudging the keyring until it slid right over the welcome mat. Can you guess what secret treasure was hiding right inside the door?