The sink

is a reservoir...

...for all the rainwater on the island. At low tide, the water level inside the island is higher and the water can flow "out" of the island through a valve in the pumping station.

To the riddle

On the way to the Spühlkuhle you will pass the AMT PELLWORM. Count the bricks above the window.

Make a stopover at the island museum or at the spa and tourist center . Both are also a few meters away.

Now form a combination of the number of stones and the playing card with the sailing ship