An anonymous reader quotes a report from The Guardian:"The majority of them use the drop valve system, which sits at the bottom of the cistern and opens to allow the water to flow out when the flush is pressed," reports The Guardian. "Debris and other issues can cause the mechanism to fail to close, meaning water continually flows from the cistern and into the bowl.""Older cisterns tend to use a siphon system, which works by pushing water upwards until it reaches a point at which it can flow into the bowl. Because that point is above the waterline when the toilet is not being flushed, the system is less likely to develop the same sort of leak." Jason Parker, the managing director of Thomas Dudley Ltd, one of the UK's largest plumbing manufacturers, told the BBC: "If we're serious about wasting water and we want to stop it, the only way to do that is put a siphon back in."