As the weather gets hotter and people can spend more time outside and exercising, it is important to stay hydrated.
Water regulates temperature in the body, transports nutrients in the blood, removes waste through urine and acts as a lubricant and shock absorber in joints.
You lose water when you go to the toilet, when you sweat but also when you breathe, and it must be replaced regularly.
Even one per cent dehydration can affect both your physical and mental function, like concentration, while someone with mild dehydration can suffer headaches.
Thirst can make you feel hungry too, so you may end up snacking or eating more than you should as part of a healthy diet.
With all of this in mind, you should aim to drink six to eight glasses of fluid a day. And if the weather is hot or you are exercising, you may need more than that.
Water is a cheap and healthy choice for quenching your thirst. It has no calories and contains no sugars that can damage teeth. But there are other options, too.
Tea and coffee count as fluids, as does milk, which contains nutrients such as protein, B vitamins, iodine and calcium. A low-fat milk such as semi-skimmed or skimmed is recommended.
Fruit juices and smoothies also give you vitamins and minerals in addition to hydration, but they contain sugar too and can damage your teeth so you should limit yourself to a total of 150ml a day.
Fizzy drinks and energy drinks are best avoided as they are high in sugar, while alcohol causes you to lose more water in your urine, which may lead to dehydration.
See: NHS on hydration