Many very good replies here. A couple points:
Someone asked what a "mOsm" is.........
Now, for those of you describing a lot of sweating & replenishing yourselves with water..... Sweat contains more water than electrolytes, but both are lost. So, you truly do become "dehydrated," but you were also "volume depleted" (your body volume is directly related to electrolyte composition, namely sodium, not water). If you had measured the sodium concentration in the blood, it would've been high. Now you have two very intense stimuli of the thirst reflex: (1) dehydration & (2) volume depletion. If you correct the dehydration component by only drinking water, you can bring your body's water balance back in check. But, since you haven't taken in the electrolytes, which is what drives your body volume, the thirst reflex isn't shut off....instead you keep drinking, and drinking. Eventually you'll repair the volume deficit by force, but now you've gone from being dehydrated to overhydrated in a short period of time. This swing in your body's electrolytes (specifically, sodium) can be quite dangerous, and neurological symptoms are what predominate.
Now, suppose you had taken in an electrolyte-containing solution from the start. You then repair both the water deficit AND the solute deficit, which solves everything.......
Oh, and to respond to the person that wanted to color her pee pee, lots of medications will do it....but go eat some beets. You may be one of the folks lacking in the enzyme to break down betaine, and you'll end up with red pee pee.