In 1954 a government directive issued the announcement that we should all take in around 8 glasses of fluid a day and so the myth was born - as people automatically equated fluid to being actual water. However what was actually meant was that we should take in fluid from various sources. Some foods have a high water content (including drinks like tea and coffee). Note though that drinks containing milk also contain protein and this can sometimes detract from water intake. It has been noted that the slogan 'if you are thirsty it's too late' reads more like an advertising campaign than anything else and so the real advice is this: If you are thirsty then have a drink - if not then don't.