We are regularly advised to drink more water: it clears skin, reduces tiredness and aids concentration.
But the death of a woman in the US after taking part in a water-drinking contest shows you can have too much of a good thing.
Jennifer Strange had taken part in the "Hold Your Wee for a Wii" game, which promised the winner a Nintendo Wii. Afterwards she reportedly said her head was hurting and went home, where she was later found dead. Initial tests have shown her death is consistent with water intoxication.
Drinking too much water can eventually cause your brain to swell, stopping it regulating vital functions such as breathing, and causing death. So what happens?

A feature to the BBC News Magazine - aiming to answer some of the questions behind the headlines
Water enters the body when we drink and is removed primarily in the urine and sweat. The amount of water in the body is regulated to control the levels of certain compounds, such as salt, in the blood.
If you drink too much water, eventually the kidneys will not be able to work fast enough to remove sufficient amounts from the body, so the blood becomes more dilute with low salt concentrations.
"If you drink too much water it lowers the concentration of salt in your blood so that it is lower than the concentration of salt in cells," says Professor Robert Forrest, a consultant in clinical chemistry and forensic toxicology at the Royal Hallamshire Hospital in Sheffield.
The water then moves from the dilute blood to the cells and organs where there is less water. Professor Forrest likens this to the effects seen in science-class experiments.

Actor Anthony Andrews was treated for water intoxication in 2003
"If you put salty water on onion skin the cells will shrink, if you put too much water on it the cells will swell," he says.
This swelling is a problem in the brain.
"When the brain swells, it is inside a bony box so has nowhere to go," he says. "The pressure increases in the skull and you may get a headache. As the brain is squeezed it compresses vital regions regulating functions such as breathing."
Eventually these functions will be impaired and you are likely to stop breathing and die. Warning signs included confusion and headaches.
Symptoms would normally occur very soon after drinking the water, but if the gut is absorbing the water more slowly then it can take longer.
Drinking several litres over a relatively short period of time could be enough to cause water intoxication. Those most at risk include people taking ecstasy, as the drug increases thirst and facilitates the release of anti-diuretic hormones so more water is taken in but cannot be excreted. Also, elderly people because their kidney function may be impaired.
Treatment for drinking excess water is "relatively straightforward", says Professor Forrest. It includes giving patients diuretics to help decrease their water load, or using drugs to reduce the swelling caused by excessive water.