The 2 litre thing is a frequent misquoting of a study that (I think) was done in 1945- it found that the human body needed 2 litres of water a day, but this didn't necessarily have to be drunk, because there's a lot of water gained through food etc. Basically, you need to drink something every day, but just don't worry about it- if you drink every time you feel thirsty you're probably drinking enough.