How many calories are gained by drinking a glass of water?

The correct answer is A) Zero. Plain water contains no carbohydrates, fats or proteins, and those are the only sources of food energy. Energy content is calculated from macronutrients using Atwater factors: about 4 kcal per gram of carbohydrate or protein and 9 kcal per gram of fat. Water has none of these, so it adds no calories.

Why the other options are wrong:

• B) 15, C) 25, D) 50: These figures are too high for plain water. Any calories in a drink come from dissolved sugars or other additives, such as sweetened juice, cola or flavoured squash. Water itself is not the source.

Core concept: energy balance and the caloric value of macronutrients. This links to the General Science and Health syllabus and to Mains questions on obesity and lifestyle diseases, where sugary beverages are a common policy focus.

Trap: Candidates often count flavoured or sweetened drinks, or sports drinks, as 'water'. Only plain water is zero-calorie.