We’ve all heard various theories related to hydration. Drink eight glasses of water a day. Drink every 20 minutes while exercising. If your muscles cramp or your pee is dark yellow, you’re dehydrated. Don’t drink too much or you could get hyponatremia. The problem is that the rules of hydration seem to change regularly. Here’s what you need to know: the amount of water you need to stay hydrated depends on many factors, and there’s no prescribed amount of fluids or frequency of consumption that’s right for everyone across all conditions. We asked Dr. Steve Halvorson, an emergency doctor who serves as medical director of the Hardrock 100, for his take on how to spot dehydration (whether or not you're exercising) and what to do about it.