Secret Piece is an item in the game.
There are 6 of them in total, which can be found in the secrets of the zones.
Sources[]
Secret Piece I[]
Found in the Island.
During the Fishtival in Chapter 8-2, put tiles 
12 and 
18 together so that they form a lake and fish there. You will catch a puzzle piece of the Airship.
Place the tiles like this.
Secret Piece II[]
Found in the Grassland.
In Chapter 2, at the beginning of the quest Finding the Ghost Lily, when Monkar wakes you up to go catch the Ghost Lily, go outside the tent to the tile 
15, and find the plant pot next to the tent. After interacting with it 5 times, a flower will bloom, and you get a puzzle piece of the Airship.
Secret Piece III[]
Found in the Forest.
In Chapter 6-2, in tiles 
5, 
8, 
12 and 
16 there is a bird on one corner of each tile. Place the tiles together so that the 4 birds are close to each other. Go inspect the birds, they will drop a puzzle piece of the Airship.
The bird in one of the tile.
Secret Piece IV[]
Found in the Desert. In the Desert, you will find plenty of cactus plants. Interacting with them 5 times to get a puzzle piece of the Airship.
There are a lot of cacti, touch them several times.
Secret Piece V[]
Found in the Volcano. After you make the volcano erupt, rapidly rotate the volcano's tile for it to create a flame tornado. You will then receive a puzzle piece of the Airship.
Rotate this tile quickly to get a puzzle piece.
Secret Piece VI[]
Found in the Iceberg. Go find the pieces of the Snowman:
- Snowball in tile 4.
The Snowball's location.
- Branches in tile 4.
The branches' location.
- Beanie in tile 6. This one requires ice sliding.
Now go back to tile 
4 and assemble the Snowman.
After you finished the snowman, you will receive a puzzle piece of the Airship.
Usage[]
6 puzzle pieces are used to unlock the secret zone Airship in the main menu.