Abstract: A fluid displacement device (2) comprises a flexible plastics container (4). The container (4) is formed from a single sheet of PET/PE which is folded over to create a pocket, and heat sealed along the seams 8a,b,c formed by the edge of the sheet. The container (4) is substantially rectangular in shape, and measures approximately 130 mm by 200 mm. The container is placed in a toilet cistern and suspended above a bottom part thereof. Water flows into the flexible container (4) from the cistern (20) through the holes (6) and comes into contact with the water-swellable polymer. The water-swellable polymer absorbs the water and swells to approximately (400) times its original weight to displace approximately 500 ml water from the cistern (20).