Don't drink eight glasses of water a day - do this instead.
We can't, much as some would like to, live off water and sunshine although we do need both to survive. But how much exactly?
A new study has found that our bodies have inbuilt functions that carefully calibrate the amount of water we need and that 'rules' are often wrong.
There's the rule of three, which is a pretty good guide; we can generally survive for three minutes without oxygen, three days without water and three weeks without food.