The product is a minimum two-layer composite, preferably made of paper. The composite layers (1,2) comprise a sufficiently strong material with good water absorbence and between them there is a pouch (3) sealed around the circumferential edge. The chamber contains a substance (4) which liberates a gas when it comes into contact with water. A similar product containing a gas in the chamber instead of a gas-liberating substance may also be used. The gas-liberating substance is preferably a mixture of sodium bicarbonate and citric acid.
