Water brash is the sudden flooding of the mouth with thin, acidic fluid regurgitated from the stomach — typically accompanied by heartburn and a sour taste. It is a symptom of gastro-oesophageal reflux: the valve between stomach and oesophagus relaxes inappropriately, letting acid and partially digested contents rise. Large meals, fatty or spicy foods, alcohol, coffee, smoking, and lying down after eating all provoke it; pregnancy and obesity raise the risk. Occasional water brash is common and managed with smaller meals, weight control, and avoiding personal triggers; frequent episodes suggest reflux disease and deserve medical assessment, since chronic acid exposure damages the oesophagus. Antacids give short-term relief.