Too late

Sometimes people say things that don't make a whole lot of sense.

You ever heard this one...

"Drink plenty of water, because by the time you feel thirsty, it's already too late."

Too late!? For what!?

Am I going to die because I haven't had a glass of water? I'm not trekking through the sands of the Sahara. Am I going to expire between my desk and the sink? If so, I better put some pants on.