There are 5 Tingle Statues hidden away in 5 of the Game's Dungeons. In the original GCN Game, you could uncover them by using a Tingle Bomb via the Tingle Tuner, an item Tingle gives you when you spring him from jail. That item was replaced with the Tingle Bottle, so you must use Normal Bombs to uncover the Statues now. When you get all 5, Knuckle finally appears on Tingle Island.