Abstract: A display device comprises a sealed flat container formed by two rectangular sheets of glass 10,11 spaced from each other by a small amount and sealed together by a glue layer 12 at their outer edges. The container encloses water and detergent, a quantity of air and fine sand 26. After settling of the sand in one direction, the container is inverted causing bubbles 27 of the air to rise and form a bubble layer partially supporting the sand while allowing it to percolate gradually downwardly between the bubbles.