This falls short of the recommendation from the Food Standards Agency in the UK, which says that people living in moderate climates should drink at least six to eight glasses of water or other fluid to prevent dehydration.

The Mayo Clinic in the US recommends that we drink more during increased physical activity, hot and humid weather, when indoor air is dry or while at high altitudes, or during an illness, particularly one accompanied by fever, vomiting or diarrhoea.

Water is essential for life: while well-fed humans can survive for several weeks without food, we can't live for longer than a week at the most without water. We need it for so many bodily functions: from regulating body temperature, transporting nutrients and oxygen to body cells and taking waste products and toxins out of the body, to lubricating joints and body tissues such as mouth, eyes, nose and to make up blood, saliva, stomach acids and urine.

On average we lose about 10 cups of water a day as a result of sweating, urinating, breathing and bowel movements. Our bodies are very sensitive to the loss of water: a 1 per cent loss of body water will trigger sensations of thirst, 2-5 per cent loss will lead to a dry mouth, lightheadedness, tiredness and headache. Any more than an 11 per cent loss of body water could lead to death.

To prevent this from happening, you need to keep hydrated. Drinking water is the best and cheapest way to replace lost fluids. But other beverages such as milk, soy milk, clear soup, and fruit juice also count, as do foods such as watermelon and tomatoes, which are more than 90 per cent water.

Tea and coffee aren't very good options for hydration, as they have a diuretic effect which means they cause your body to lose more water by increasing the number of times you go to the toilet.

If you're not sure whether you're drinking enough, check your urine. A small quantity of dark-coloured urine means you're not drinking enough. Drink more so your urine is pale and nearly colourless.

Tips for drinking more

Buy a funky bottle to fill with water and take it with you.

Keep a glass of water next to you when you're working at your desk, and take regular sips while you're doing homework or on the computer.

If you don't like the taste of water, try adding a slice of lime or lemon, or a sprig of mint and some ice to your water. These give plain water a totally different, refreshing taste.

Quench your thirst and hunger by chomping on ice-cold watermelon slices or juicy tomatoes.

Fruit smoothies make a refreshing change from caffeinated drinks - and help you get some of your five fruit and veg a day, too!

Try starting a meal with a soup such as consomme or minestrone, or have a bowl as a comforting snack when you get home from school.

Vincy's diary:

Breakfast: Oatmeal, cheese and vegetable sandwich, or an egg, an apple or orange

Lunch: Rice, vegetables, port

Dinner: Rice porridge, fish, carrots, apple, tomato

Snacks: Soya milk, ice cream

Exercise: 15 minutes jogging three times a week, two hours swimming a week