Why do gnocchi and German potato klösse swim in boiling water when they finished cooking?

Germany

Gnocchi and German potato klösse are made of hydrous and hydrophilic ingredients such as flour, bread, potatoes and eggs. In uncooked condition the specific weight is higher than water, that’s also the reason why they sink to the bottom. At first contact with the boiling water, the gnocchi or klösse start to cook on the outside where the starch and gluten agglutinate. After some time the inside also starts to cook. The enclosed water evaporates and the air expands, but the air can’t disappear because the surface is already impermeable. Therefore the gnocchi and klösse will be aerated. Their volume starts to increase (through displacement of water), but their weight remains the same – the density decreases, and as soon as it gets lower than the density of water they start to float.